746 



CHORDATA. 



sp.c 



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 subsequently 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 early distinguishable, being 

 imbedded 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. 405 

 C and 410, po). They appear to be 

 derived from the epithelial cells 

 amongst which they lie ; and are 

 characterized by containing a large 

 granular nucleus, surrounded by a 

 considerable body of protoplasm. 

 The peritoneal epithelium in which 

 they are placed is known as the 

 germinal epithelium. 



It is at first impossible to distin- 

 guish the germinal cells which will 

 become ova from those which will 

 become spermatozoa. 



The former however remain with- 

 in the peritoneal epithelium (fig. 41 1), 

 and become converted into ova in a 

 manner more particularly described 

 in Vol. II. pp. 54 59. 



FIG. 410. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM 

 EMBRYO SLIGHTLY YOUNGER 

 The history of the primitive THAN 28 Ft 

 germinal cells in the male has not #' spinal .cord; w. white 

 been so adequately worked out as in 

 the female. 



matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 



,p u - mp. muscle-plate ; tup', inner layer 



1 he lullest history of them is of muscle-plate already converted 

 that eiven bv Semnpr fNn ^<\\ fr.i- into muscles J Vr - rudiment of 

 u T?I * ' vertebral body; st. segmental 



the -blasmobranchii, the general ac- tube; sd. segmental duct; sp.v. 



rnrarv rf tirViiVV. T ^ ^ f n spiral valve ; v. subintestinal vein ; 



uracy Of which I can fully support ; ,. primitive generative cells. 



