264 DEVELOPMENT OF ELASMOBRANCH FISHES. 



direction. The growth of the blastoderm has continued to be 

 very rapid. 



In the region of the embryo (PI. 7, fig. 9) more important 

 changes have occurred. The epiblast still remains as a single 

 row of columnar cells. The hypoblast is no longer fused with 

 the mesoblast, and forms a distinct dorsal wall for the alimentary 

 cavity. Though along the axis of the embryo the hypoblast is 

 composed of a single row of columnar cells, yet in the lateral 

 part of the embryo its cells are less columnar and are one or 

 two deep. 



Owing to the manner in which the mesoblast became split 

 off from the hypoblast, a continuity is maintained between the 

 hypoblast and the lower layer cells of the blastoderm (PI. 7, 

 fig- 9)> while the two plates of mesoblast are isolated and dis- 

 connected from any other masses of cells. 



The alimentary cavity is best studied in transverse sections. 

 (Vide PI. 7, fig. ioa, lob and loc, three sections from the same 

 embryo.) It is closed in above and at the sides by the hypoblast, 

 and below by the yolk. In its anterior part a floor is commencing 

 to be formed by a growth of cells from the walls of the two 

 sides. The cells for this growth are formed around the nuclei 

 of the yolk ; a feature which recalls the fact that in Amphibians 

 the ventral wall of the alimentary cavity is similarly formed in 

 part from the so-called yolk cells. 



We left the mesoblast as two masses not completely sepa- 

 rated from the hypoblast. During this stage the separation 

 between the two becomes complete, and there are formed two 

 great lateral plates of mesoblast cells, one on each side of the 

 medullary groove. Each of these corresponds to a united 

 vertebral and lateral plate of the higher Vertebrates. The plates 

 are thickest in the middle and posterior regions (PI. 7, fig. ioa 

 and iob], but thin out and almost vanish in the region of the 

 head. The longitudinal section of this stage represented in PL 7, 

 fig. 9, passes through one of the lateral masses of mesoblast cells, 

 and shews very distinctly its complete independence of all the 

 other cells in the blastoderm. 



From what has been stated with reference to the develop- 

 ment of the mesoblast, it is clear that in Elasmobranchs this 

 layer is derived from the same mass of cells as the hypoblast, 



