iSz Edwin G. Conklin. 



gastrulation a blastopore groove is left in the posterior half of the 

 embryo, on each side of which lie the muscle cells. (Fig. 9.) By 

 the continued growth of the anterior lip this groove is shoved to 

 the posterior end of the embryo and the rows of muscle cells are 

 tilted up from an antero-posterior to a vertical position. Later, 

 when the notochord is formed, the muscle cells come to lie alongside 

 of it, thus forming the three rows of muscle cells on each side. 

 Finally the ectoderm of the posterior lip of the blastopore, which 

 has, up to this stage, formed a notch at the end of the blastopore 

 groove, grows forward and reduces this groove to a minute pore. 



Owing to the absence of the anterior lip of the blastopore, and 

 of the notochord and the neural plate, the later stages in the develop- 

 ment of these posterior half embryos is much altered. In the first 

 place the blastopore groove and the muscle cells are not pushed to 

 the posterior end of the embryo. Then the muscle cells on each 

 side of the blastopore groove are not kept apart by the notochord 

 but come into contact forming a continuous layer of muscle cells 

 across the dorsal side. (Fig. 56.) The blastopore groove, therefore, 

 disappears by the fusion of the lateral lips of the groove and the 

 ectoderm cells grow over the whole dorsal surface; the only trace 

 of the blastopore groove which is left is a slight notch in the ante- 

 rior border of the embryo. (Figs. 55, 56.) The ectoderm never 

 entirely incloses the posterior half embryo on the side next the 

 injured cells, but the endoderm here comes to the surface as shown 

 in Figs. 57 and 58. 



No trace of notochord, neural plate nor sense spots ever appears 

 in these posterior half embryos, and what is more remarkable a 

 tail is never formed but the embryo always remains rounded in 

 form, as shown in Figs. 55-58. It is quite evident that the elonga- 

 tion of the tail of the normal larva, together with the elongation of 

 the individual muscle cells and perhaps also the arrangement of 

 these cells in three rows on each side, is dependent upon the pres- 

 ence and elongation of the notochord. Perhaps one reason why 

 a normal notochord is never formed in the anterior half embryo 

 is due to the fact that the ectoderm does not completely inclose the 

 embryo, so that the chorda cells in their growth crowd out of the 

 open side and hence become free and scattered. 



In conclusion, the study of anterior or posterior half embryos 

 establishes in a most convincing manner the fact that the develop- 

 ment of individual blastomeres of the ascidian egg is a mosaic work. 



