ELASMOBRANCHII. 51 



of separate segments, one of which is shewn in fig. 24, pp. The 

 walls of the segments eventually give rise to the main muscles of 

 the branchial clefts, and probably also to the muscles of the 

 mandibular arch, of the eye, and of other parts. The cephalic 

 sections of the body cavity will be spoken of as head cavities. 



In addition to the parts 

 already mentioned the meso- 

 blast gives rise to the whole 

 of the vascular system, and 

 to the generative system. 

 The heart is formed from 



part of the splanchnic meso- FlQ ^ HORIZONTAL SECTION THROUGH 

 blast, and the generative THE LAST VISCERAL ARCH BUT ONE OF AN 



r r il_ EMBRYO OF PRISTIURUS. 



system from a portion of the 



mesoblast of the dorsal nart e ?' e P iblast 5 vc - P ouch of hypoblast 

 sai pare which win form the walls of a visceral cleft . 



of the body cavity. pp- segment of body-cavity in visceral arch ; 



mt- i- TT aa. aortic arch. 



The hypoblast. Very 



shortly after the formation of the mesoblastic plates as lateral 

 differentiations of the lower layer cells, an axial differentiation 

 of the hypoblast appears, which gives rise to the notochord very- 

 much in the same way as in Amphioxus. 



At first the hypoblast along the axial line forms a single 

 layer in contact with the epiblast. Along this line a rod-like 

 thickening of the hypoblast very soon appears (fig. 25, B and C, 

 Ch'} at the head end of the embryo, and gradually extends back- 

 wards. This is the rudiment of the notochord ; it remains 

 attached for some time to the hypoblast, and becomes separated 

 from it first at the head end of the embryo (fig. 25 A, ch} : the 

 separation is then carried backwards. 



A series of sections taken through an embryo shortly after the first 

 differentiation of the notochord presents the following characters. 



In the hindermost sections the hypoblast retains a perfectly normal 

 structure and uniform thickness throughout. In the next few sections (fig. 

 25 C, Ch'} a slight thickening is to be observed in it, immediately below the 

 medullary groove. The layer, which elsewhere is composed of a single row 

 of cells, here becomes two cells deep, but no sign of a division into two 

 layers is exhibited. 



In the next few sections the thickening of the hypoblast becomes much 

 more pronounced ; we have, in fact, a ridge projecting from the hypoblast 

 towards the epiblast (fig. 25 B, Ch'}. This ridge is pressed firmly against 



42 



