242 W. JR. Coe — Development of the 



the egg where the polar bodies were extruded) become more drawn 

 out and flattened (Fig. 1, PI. xxxii), while those of the opposite pole 

 become more columnar in shape and extend far inward into the seg- 

 .mentation cavity. The blastula flattens out and tHe lower half begins 

 a regular invagination. 



Before invagination, however, a few cells which are to form the 

 larval mesoderm are separated off from the endoderm cells and make 

 their way into the segmentation cavity (Fig, 1, PI. xxxii). The actual 

 origin of these cells is difficult to make out in 31. cmca. In C. lacteus 

 and C. marginatus, where the blastomeres are not of so nearly the 

 same size, the origin of the mesoderm is more easily determined and 

 will be discussed in a separate paragraph. Suffice it to say here that 

 it appears to arise from two somewhat distinct, though closely con- 

 nected, sources — from the divisions of a large, posterior pole cell, as 

 in annelids, and from some of the endoderm cells. These latter seem 

 to be indiscriminately pushed inward and set free in the segmentation 

 cavity. It is possible that this appearance is misleading and that all 

 of the mesoderm is actually derived from a single polar cell. The 

 derivatives of the polar mesoderm cell always lie primarily in the 

 lower wall of the blastula between the ectoderm and the entoderm. 

 After the cells enter the segmentation cavity they lie upon the ento- 

 derm, and after gastrulation arrange themselves around the mouth 

 of the gastrula (Fig. 4, PI. xxxii). 



As gastrulation proceeds the bilateral nature of the gastrula 

 becomes very evident, not only by the arrangement of the super- 

 ficial cells, but also by the marked backward inclination of the 

 invaginating layer of entoderm (Fig. 1, PI. xxxiii). In those species 

 which contain but little yolk (ilT. cceca, C. leidyi) the gastrulation is 

 perfectly regular and the entoderm-cells retain nearly the same rela- 

 tions with each other that they held in the blastula. Where yolk is 

 abundant ( C. lacteus, C. marginatus) the arrangement of the cells is 

 broken up by the invaginating process, and they assume new posi- 

 tions in the enteron. 



Development of the Pilidmm. 



By the end of the first day the cells of the dorsal surface of the 

 embryo become much more flattened and their boundaries much less 

 evident. Most of the yolk-globules originally contained in the cells 

 have meanwhile been absorbed and the ectoderm becomes almost 

 perfectly transparent. At the time of the absorption of the yolk 

 many of the cells contain one or more large, clear vacuoles which 



