THE DEVELOPMENT OF THE SILP^. 533 



The egg is impregnated in the ovarian follicle, as in Pyro- 

 soma ; and the oviduct, shortening, gradually draws the ova- 

 rian follicle, with its contents, into a sort of incubatory pouch, 

 which is a diverticulum of the wall of the atrium, and pro- 

 jects into the atrial cavity. 



For distinction's sake the incubatory pouch may be termed 

 the ovicj/st. As the oviduct shortens, it widens, and consti- 

 tutes, together with the ovarian follicle, a single uterine sac^ 

 the outer or oviducal half of which applies itself to the wall 

 of the ovicyst, while the inner half contains the ovum. The 

 vitellus undergoes complete division, and the superficial layer 

 of blastomeres constitutes itself into an epiblast, investing 

 the solid mass formed by the other blastomeres, which repre- 

 sent the hypoblast. A mesoblastic layer subsequently ap- 

 pears between the two. The nervous gangUon results from 

 an involution of the epiblast, while the branchial sac, the 

 alimentary canal, and the asrium, are the product of the sub- 

 division of a cavity which appears in the midst of the hypo- 

 blast. The maternal and the foetal parts of the placenta 

 arise, respectively, from the wall of the ovarian sac, and from 

 certain large blastomeres on the adjacent haemal face of the 

 embryo. 



Todaro agrees with other observers in stating that the 

 vitellus undergoes division, and that a small celled blastoderm 

 invests the large remaining cells, which he terms the germinal 

 mass. But his account of the further stages of development 

 is very different. A circular thickening of the blastoderm 

 separates the hemisphere which is directed outward from that 

 which is turned inward, and gives rise to a lamellar outgrowth. 

 It is, at first, directed toward the inner end of the ovisac, 

 having reached the bottom of which, it becomes reflected ; 

 and the reflected portions lining the inner wall of the ovisac, 

 and meeting over the outer hemisphere, form a sort of am- 

 niotic investment of the embryo. It is the cavity left be- 

 tween this " amnion " and the inner hemisphere of the blasto- 

 derm, which becomes the parental blood-sinus. An involu- 

 tion of the outer hemisphere of the blastoderm gives rise to 

 the alimentary canal, which becomes shut off, as the endoderm, 

 from the remaining blastoderm, which constitutes the ecto- 

 derm. A mass of cells which appears in the middle of the 

 outer half of the embryo, between the alimentary sac and the 

 ectoderm, and which has only a transitory existence, is re- 

 garded by Todaro as the representative of the urochord. 



