Experimental Studies on Germinal Localization. 33 



shaped. There can, I think, be no doubt that these are meso- 

 blast cells,^ though I have not determined whether they are the 

 products of the second somatoblast, 4d, or arise from another 

 source. A possibility of error on this point is given by the fact, 

 already referred to, that just anterior to the prototroch on either 

 side are two lateral ectoblastic areas of proliferation (of unknown 

 significance) that may contribute to the small cells in question. 

 In any case these lateral masses of mesoblast fail to appear in the 

 lobeless embryos of corresponding age or older. . In the earlier 

 stages, of which Fig. 33 is an example, it is impossible to de- 

 termine this point with any degree of certainty, owing to the 

 crowding together of the entoblast cells in a compact mass in 

 which frequently no cavity can be seen. In later stages, however, 

 both longitudinal and transverse sections give pretty clear evi- 

 dence that the small mesoblast-cells are either wholly absent or 

 very few in number. Fig. 40 is from a complete series of lon- 

 gitudinal sections of a lobeless embryo of 30 hours. This shows 

 the gut as a two-chambered sac directly applied to the ectoblast 

 with no sign of smaller cells between them, though both the 

 anterior ectoblastic areas of proliferation are shown (a, a). It 

 might well be supposed that the small cells are present in a dif- 

 ferent plane, as would be the case in Fig. 38 if cut in the sagittal 

 plane ; but their absence appears no less clearly in cross-section, as 

 shown in Fig. 41 (from a complete transverse series). This 

 embryo of 48 hours swam actively and normally. Though not 

 so well fixed as the preceding one, it clearly shows the gut as a 

 simple sac, enclosing a single cavity that opens at the posterior pole 

 and anteriorly is nearly filled with a thickening bulging inward 

 from the wall at one side. I am quite sure that no mesoblast- 

 cells are present in this embryo unless at the extreme anterior end, 

 where the layers are cut tangentially and cannot be clearly an- 

 alyzed. The sections of this embryo clearly show further 



1 The relations as figured by Kowalewsky ('83, Fig. 48) in the Marseilles 

 species are essentially similar to those here shown, except that the mesoblast- 

 cells are shown very much larger and fewer. This is stated to be from a larva 

 of 24 hours, but probably represents a relatively earlier stage of development 

 than mine. Compare the mesoblast-cells in Kowalewsky's Fig. 66, from a larva 

 of 38 hours. 



