Hemichordate

P. flava’s early cleavage pattern is similar to that of S. kowalevskii. The first and second cleavages from the single cell zygote of P. flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. P. flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and 4 larger macromeres. Further divisions occur until P. flava finishes the blastula stage and goes on to gastrulation. The animal mesomeres of P. flava go on to give rise to the larva’s ectoderm, animal blastomeres also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues.[15] Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development P. flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage.[16]

Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm[17][18]

Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii, and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4, which is homologous to Drosophila’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on the ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii, such as a netrin that groups with netrin gene class 1 and 2.[6] Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.

A phylogenetic tree showing the position of the hemichordates is:

The internal relationships within the hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[23][24][25]