ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 107 
the first 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, but 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 Strongy/ocentrotus, 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 
ege 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 сопсгевепсе 
of the posterior lip of the blastopore, the bases of the sub-oesophageal ganglia are 
carried to the ventral side. Тһе nervous system of annelids and mollusks thus has 
& 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. In 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 | 
