DEVELOPMENT OF PETROMYZON FLUVIATILIS. 183 



Lepidosteus, Spencer in the Frog, and Miss Johnson 

 in the Newt, 



The further development of the central nervous system will 

 be described later after some of the details connected with the 

 mesoblast and hypoblast have been considered. 



The Mesoblast, 



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

 angle between the epiblast and the invaginated endoderm has 

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

 mains 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 

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

 mesoblast soon unites ventrally by lateral downgrowths; in 

 the trunk, however, which remains crowded with yolk- cells for 

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

 later, 



Scott has described the formation of the muscle-plates very 

 accurately, and it will therefore be unnecessary to give more 

 than a short resume in order to make the following account 

 intelligible. About the twelfth or thirteenth day the meso- 

 blastic somites appear by the segmentation of the dorsal part 

 of the lateral mesoblastic plates. These appear at first ante- 

 riorly, and the segmentation extends backwards. The most 

 anterior one lies close behind the auditory sac. The ventral 

 unsegmented mesoblast has split into the splanchopleure and 

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

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

 mesentery, connecting the alimentary canal with the ventral 

 body wall. 



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

 sents a horizontal section through an embryo fourteen days 

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

 often entirely obliterated, which represents part of the body 

 cavity. The cells surrounding this are at first uniform in size, 

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



