42 THE HYPOBLAST. 



of the mesoblast in the trunk. This division causes the body-cavity 

 of the head to be divided up into a series 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 

 /sf-sf- -|^^^^^^^ cephalic sections of the body- 



cavity will be spoken of as 



T1 ^ „. XT head cavities. 



Fig. 24. Horizontal section thkough _ \^•^.• + +t^ +1 



THE LAST VISCEKAL AKCH BUT ONE OF AN ^^ aClClltlOn tO tUe partS al- 



EMBRYo OF Pristiurus. Tcady mcntioned the mesoblast 



ep. epiblast; vc. pouch of hypoblast gives rise to the whole of the 



which will form the walls of a visceral cleft; v^SCular system, and to the 

 pp. segment of body-cavity m visceral arch ; , . -^ ' n^^ ^ 



«a. aortic arch. generative system. 1 he heart 



is formed from part of the 

 splanchnic mesoblast, and the generative system from a portion of the 

 mesoblast of the dorsal part of the body-cavity. 



The hypoblast. Very shortly after the formation of the meso- 

 hblastic plates as lateral differentiations of the lower layer cells, an 

 jjaxial differentiation of the hypoblast appears, which gives rise to the 

 'inotochord 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, CJi) at the 

 head end of the embryo, and gradually extends backwards. 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 

 difi'erentiation 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, Gh') 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 

 the epiblast, and causes in it a slight indentation. The hypoblast in the 

 region of the ridge is formed of two layers of cells, the ridge being entirely 

 due to the uppermost of the two. 



In sections in front of this a cylindrical rod, which can at once be 

 recognised as the notochord, and is continuous with the ridge just described, 

 begins to be split off from the hypoblast (fig. 25 A, Ch). It is difficult to 

 say at what point the separation of this rod from th<> hypoblast is com- 



