614 



CHORD AT A. 



becomes differentiated during the segmentation. This cell, which is 

 in close contiguity with the cells from which both the mesoblast and 

 hypoblast originate, subsequently divides ; but at the gastrula stage, 

 and after the mesoblast has become formed, the cells it gives rise to 

 are enclosed in the epiblast, and do not migrate inwards till a later 

 stage. The products of the division of the generative cell sub- 

 sequently divide into two masses. It is not possible to assign the 

 generative cell of Moina to a definite germinal layer. Grobben, 

 however, thinks that it originates from the division of a cell, the 

 remainder of which gives rise to the hypoblast. 



Chordata. In the Vertebrata, the primitive generative cells 

 (often known as primitive ova) are earlv distinguishable, being im- 

 bedded amongst the cells of two linear 

 streaks of peritoneal epithelium, placed 

 on the dorsal side of the body cavity, 

 one on each side of the mesentery (figs. 

 4<()5 C and 410, po). They appear t<> IM> 

 derived from the epithelial cells amongst 

 which they lie; and are characterized 

 by containing a large granular nucleus, 

 surrounded by a considerable body of pro- 

 toplasm. The peritoneal epithelium in 

 which they are placed is known as the 

 germinal epithelium. 



It is at first impossible to distinguish 

 the germinal cells which will become ova 

 from those which will become sperma- 

 tozoa. 



The former however remain within 

 the peritoneal epithelium (fig. 411), and 

 become converted into ova in a manner 

 more particularly described in Vol. I. pp. 

 4347. 



The history of the primitive germinal 

 cells in the male has not been so ade- 

 quately worked out as in the female. 



The fullest history of them is that 

 given by Semper (No. 559) for the 



Elasmobrauchii, the general accuracy of of muscle-plate already converted 

 which I can fully support; though with into muscles; Vr. rudiment of 

 reference to certain stages in the history 

 further researches are still required 1 . 



In Elasmobranchii the male germinal 

 cells, instead of remaining in the germinal 

 epithelium, migrate into the adjacent stroma, accompanied I believe 



Stf' 



-sp.v. 



FIG. 410. SECTION THROUGH 

 THE TKUNK OF A ScTLLIUM EMBRYO 

 SLIGHTLY YOUNGER THAN 28 F. 



sp.c. spinal cord; jr. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots; ch. notochord ; 

 x. sub-notochordal rod; ao. aorta ; 

 tup. muscle-plate; mp'. inner layer 



vertebral body; st. segrnental 

 tube; .SY?. segmental duct; sp.r. 

 spiral valve; r. subiutestinalveiu ; 

 P.O. primitive generative cells. 



1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring my 

 own observations into accord with his as to the structure of the Elasmobranch testis. 



