ORGANIZATION AND (ELL-LINEAGE OF ASCIDIAN EGG. 105 



there is remarkable uniformity in the localization of the substances of the germinal 

 layers in all of these groups, the ectodermal substances being located in the upper 

 hemisphere, and the endodermal and mesodermal in the lower hemisphere of the 



egg. But in the localization of important organ bases there are many differences 

 between these phyla. 



1. Annelid-Mollusk Type. 



The pattern of localization in annelids and mollusks is very similar during the 

 cleavage stages and, so far as can be judged from present knowledge, it is much the same 

 in the unsegmented eggs of these two phyla. The fact that the ectoderm, mesoderm 

 and endoderm come from cells which are identical in origin, position and number; 

 that the umbrella, prototroch, cerebral ganglion, sub-oesophageal ganglion, mesoder- 

 mal bands, blastopore, stomodaium and intestine come from corresponding region of 

 the egg in the two groups, these facts speak strongly in favor of the regional homolo- 

 gies of the eggs of these phyla, whatever may be thought of their cell homologies 

 (Conklin, 1897; Child, 1900). But regional homologies as well as cell homologies 

 must be based upon similarities of germinal localization, and we would, therefore, 

 be justified in concluding that the types of localization were similar in the unseg- 

 mented eggs of annelids and mollusks even in the absence of any direct knowledge 

 upon that subject. But the experiments of Crampton (1896) on Illyanassa and 

 of Wilson (1904) on Dentalium as well as the observations of Lillie (1899, 1901) 

 on [/mo, and my own observations on localization in the eggs of Crepidula, Pliysa, 

 Planorbis and Limncva furnish considerable information as to the time, the manner 

 and the nature of localization in the molluscan egg during and before cleavage, while 

 numerous works on the cell-lineage of the annelids as well as the observations of 

 Wheeler (1897), Driesch (1890) and Carazzi (1904) on the unsegmented egg of 

 Myzosioma show that the nature of localization is here very similar to that found 

 in the mollusks. 



In all of these cases the only formative substances which are directly recogniz- 

 able before cleavage are those of the future germ layers. In the main the ectoder- 

 mal substances are located in the upper hemisphere and the endodermal in the 

 lower, though Wilson (1904) has found that the apical organ does not form in the 

 larva of Dentalium when the polar lobe at the vegetal pole is removed. The meso- 

 dermal substances are also located in the lower hemisphere, and since the primary 

 mesoderm cell (4d) always come from the left posterior macromere of the 4-cell 

 stage and from the posterior blastomere of the 2-cell stage it may be inferred that 

 immediately before cleavage it lies posterior to the vegetal pole; whether it may be 

 located exactly at the vegetal pole in still earlier stages and then later shift to the 

 posterior side, as in ascidians, cannot be determined at present. 



When a polar lobe is present the mesodermal substance is probably located in 

 it. Crampton (1896) found in Illyanassa that the mesoblast cell (4d) did not form 

 when the lobe had been removed; Wilson (1904) holds that in Dentalium the sub- 

 stance of the lobe is allotted to both the first and second somatoblasts'(2d and 4d), 

 and that its size is proportional to the size of those cells and of the parts to which 



14 JOUEN. A. X. S. PHILA.. VOL. XIII. 



