DEVELOPMENT OF PETEOMTZON FLTJVIATILIS. 183 



Lepidosteusj Spencer in the Frog, and Miss Johnson 

 n the Newt. 



The further development of the central nervous system will 

 DC described later after some of the details connected with the 

 nesoblast and hypoblast have been considered. 



The Mesoblast. 



The origin of this layer from the yolk-cells situated in the 

 ingle between the epiblast and the invaginated endoderm has 

 jeen described above. For some little time the mesoblast re- 

 uains in the condition of two triangular masses of cells, 

 separated from one another dorsally by the notochord and ner- 

 vous system, ventrally by the yolk-cells which lie in contact with 

 ;he ventral epiblast. In the anterior end of the embryo the 

 nesoblast soon unites ventrally by lateral downgrowths; in 

 ;he trunk, however, which remains crowded with yolk-cells for 

 I week or ten days after hatching, this takes place much 

 ater. 



Scott has described the formation of the muscle-plates very 

 iccurately, and it will therefore be unnecessary to give more 

 ;han a short re sum ^ in order to make the following account 

 ntelligible. About the twelfth or thirteenth day the meso- 

 Dlastic somites appear by the segmentation of the dorsal part 

 )f the lateral mesoblastic plates. These appear at first ante- 

 •iorly, and the segmentation extends backwards. The most 

 interior one lies close behind the auditory sac. The ventral 

 msegmented mesoblast has split into the splanchopleure and 

 ;he somatopleure on each side, and in the region just behind the 

 Dosterior gill-cleft these have met ventrally, forming a ventral 

 nesentery, connecting the alimentary canal with the ventral 



)ody wall. 



The mesoblast somites are shown in fig. 17, which repre- 

 lents a horizontal section through an embryo fourteen days 

 )ld. They are cubical masses of cells enclosing a small cavity, 

 )ften entirely obliterated, which represents part of the body 

 •avity The cells surrounding this are at first uniform in size, 

 ind each side is only one cell thick. Like the other cells of 



