ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 107 



the lirst four blastomeres. Whether the mesoplasm lies on one .side of the vegetal 

 pole in the unsegmented egg of annelids and mollusks cannot be affirmed on direct 

 evidence, but it seems not unlikely that this is the case. If this be true there is 

 here a difference between echinoderms and annelids or mollusks, in the form of 

 localization, though it is by no means impossible to derive one type from the other. 



4 . Ascidian Type. 



Finally, the type of localization in the ascidian egg differs in many respects 

 from that of the other phyla mentioned, though showing certain general resem- 

 blances to all of them and particularly to the annelid-mollusk type. Castle has 

 called attention to the fact that there are no important resemblances between 

 ascidians and annelids in their cell-lineage, and with this opinion I entirely agree. 

 Nevertheless, in the localization of ectoplasm, mesoplasm and endoplasm in the 

 unsegmented egg there are many similarities between these phyla, hut in the posi- 

 tion of specific organ bases the differences are quite notable. 



Among ascidians the ectoplasm which escapes from the germinal vesicle at the 

 animal pole does not remain there, as in the fresh-water snails, but flows rapidly 

 to the lower pole, then to the posterior side of the egg, then into the center and finally 

 into the upper hemisphere of the egg; in other phyla the ectoplasm becomes direct- 

 ly localized at the upper pole, here only indirectly. The mesoplasm is first segregated 

 at the lower pole in a manner which recalls the egg of Strongylocentrotus, and then 

 finally becomes localized on the posterior side, a result which somewhat resembles 

 the condition in annelids and mollusks ; in the ascidians the cells of the mesoder- 

 mal crescent lie in the posterior lip of the blastopore, in annelids and mollusks the 

 teloblasts and mesodermal bands lie in a similar position in the early gastrula 

 stages, but owing to the closure of the blastopore from behind forward they are 

 ultimately removed some distance from the blastopore lip. The mesoderm and 

 mesodermal organs may therefore be said to arise from corresponding regions of the 

 egg in these two groups of animals (text figs. XXXIX, XL). The endoplasm also 

 is localized in corresponding regions of the egg in these phyla. 



When, however, we come to compare the positions in the eggs of these phyla 

 of important organ bases the differences are very marked. For example, in 

 annelids and mollusks the apical plate and cerebral ganglion are formed near the 

 animal pole, the sub-oesophageal ganglia from the ventral plate, which is derived 

 from the cell 2d, lying not far below the equator on the posterior-dorsal side and 

 just above the mesodermal teloblasts (text fig. XL) ; subsequently in the concresence 

 of the posterior lip of the blastopore, the bases of the sub-oesophageal ganglia are 

 carried to the ventral side. The nervous system of annelids and mollusks thus has 

 a double origin, one portion arising from the region of the animal pole, the other 

 from the posterior pole, and these two portions subsequently become connected 

 together by commissures which surround the oesophagus. Jn the ascidian the 

 entire central nervous system is formed as a continuous plate which lies along the 

 anterior side of the egg, stretching from a point about 60 from the animal pole and 



