96 ERXEST WARREN. 



themselves in such a manner that they extend as columnar 

 cells from the perisarc to the endoderm on which the ovum 

 is still seated. 



Subsequently the ovum becomes quite central at the apex 

 of the gonangium, and is closely surrounded by the tissue 

 derived from the ectoderm of the rudimentary gonophore. 

 The endoderm is now in the form of two symmetrically placed 

 lateral lobes, one being the terminal portion of the endoderm 

 of the blastostyle (fig. 11, t.j).), and the other is the endodermal 

 spadix of the gonophore {cL C). The egg is seated symme- 

 trically between the two lobes. 



The ovum segments and a rounded cluster of about sixteen 

 loosely attached blastomeres is formed (fig*. 11, M.). The 

 cells of the endoderm immediately under the young embryo 

 (fig. 12, end.), become taller and more granular than those of 

 the rest of the endoderm. It is considered probable that the 

 nixtritive substances for the growth of the embryo are passed 

 by this endoderm into the ectodermal tissue in which the 

 embryo is imbedded. 



The blastomeres increase in number and become smaller. 

 A segmentation- cavity soon appears (fig. 13, 8g. C), and 

 this is not central, but is nearer to the apex of the gonangium. 

 Accordingly the embryo is two or three cells in thickness 

 below, while above it is mostly only one cell thick. The cells 

 of the placental tissue {jil. t.) increase in size and height ; the 

 tissvie is continuous with the thin layer of ectoderm Avhich 

 covers the sides of the blastostyle {e.B.). 



The embryo is very obviously supplied with nourishment 

 by the cells in which it is imbedded. The diameter of the 

 embryo is two and a half times that of the original ovum, 

 which never had any appreciable quantity of yolk. 



The embryo continues to expand and the constituent cells 

 increase in number and diminish in size. The segmentation- 

 cavity becomes larger (fig. 14, Bl.), A differentiation in the 

 cells of the embryo can be seen on the lower side Avhere an 

 inner layer of flattened cells now becomes evident {end.-^). It 

 may be noticed that this inner layer of endoderm lines the 



