DEVELOPMENT OF ELASMOBRANCH FISHES. 51 



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 (PL iv. 

 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. V. fig. 10 a, 10 6 and 10c, 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 (PL V. fig. 

 10 a and 10 6), but thin out and almost vanish in the region of 

 the head. The longitudinal section of this stage represented in 

 PL V. fig. 9, passes through one of the lateral masses of 

 mesoblast cells, and shows very distinctly its complete inde- 

 pendence 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, 

 and receives none of its elements from the epiblast. In connec- 

 tion with its development, as two independent lateral masses, 

 I may observe, as I have previously done\ that in this respect 

 it bears a close resemblance to mesoblast in Euaxes, as de- 



1 Quart. Journ. of Microsc. Science, Oct.. 1874. 



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