1914.] NATURAL SCIENCES OF PHILADELPHIA. 537 



In the next stags segmentation has commenced, resulting in 28 

 cells of approximately equal size. There is no sign of a blastocele 

 or any signs of differentiation. In the succeeding stage fully 100 cells 

 are present and size differences are apparent, Init while a slight 

 elongation defines the antero-posterior axis of the larva, the absence 

 of blastocele and stomodaeum renders it difficult to accurately define 

 the ventral and dorsal surfaces. 



In the following stage the differentiation of the test has com- 

 menced and, judging from two larvse where the polar bodies remain 

 attached, it extends over the greater portion of the dorsal surface and 

 to a considerable extent of the ventral as well (fig. 1, C). The 

 remaining cells, those destined to form the future animal, are thus 

 in large measure enclosed. The cells not included in the test but 

 bordering upon the surface are, generally speaking, of smaller size 

 than those upon the interior (in one specimen this is more marked 

 than in the one figured), but there is up to this time no clear differen- 

 tiation into ectoderm and endoderm. In the mid-ventral line, in 

 immediate contact with the border of the test, are slender elements 

 (st) that represent the first stages in the formation of the stomodaeum. 

 The comparatively thin cells (C, 5) in the neighborhood of the polar 

 body (not represented) are a constant feature and evidently furnish 

 the material for the development of the cerebral ganglia. 



In later stages (as in D) the various regions of the body are dis- 

 tinctly outlined, and to some extent the digestive and nervous 

 systems have been sketched in. The stomodaeum is clearly differen- 

 tiated and the mid-gut is outlined, though its constituent cells and 

 cavity are not as yet in an advanced state of development. The 

 cerebral ganglia comprise large masses of cells forming a group 

 anterior to the stomodaeum. Posteriorly, these divide, encircle the 

 stomodaeum and extend along the ventral surface to the posterior 

 end of the body. At various points in the trunk region between the 

 gut and body wall or test there are a few scattered cells, yolk-laden 

 and accordingly distinguishable from the ganglionic products. They 

 probably are mesoblastic elements. 



At the posterior end of the body is a ring of cells, ciliated in Myzo- 

 menia, that enclose a depressed area bordered in the earliest recog- 

 nizable stage by relatively slender cells. These last-named elements 

 appear to divide repeatedly and become transformed into a group of 

 cells bordering upon the surface and on the other hand passing 

 without a sharp line of demarcation into the ganglionic cords. In 

 the oldest stage represented the nerve cord appears to be completely 



