268 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Shortly after the floor of cells has appeared, the whole seg- 

 mentation cavity becomes obliterated. 



When the embryonic rim has attained to some importance, 

 the position of the embryo becomes marked out by the appear- 

 ance of the medullary groove at its most projecting part. The 

 embryo extends from the edge of the blastoderm inwards to- 

 wards the centre. 



At about the time of the formation of the medullary groove, 

 the mesoblast becomes definitely constituted. It arises as two 

 independent plates, one on each side of the medullary groove, 

 and is entirely derived from lower layer cells. 



The two plates of mesoblast are at first unconnected with any 

 other cells of the blastoderm, and, on their formation, the hypo- 

 blast remains in connection with all the remaining lower layer 

 cells. Between the embryonic rim and the yolk is a cavity, 

 the primitive alimentary cavity. Its roof is formed of hypo- 

 blast, and its floor of yolk. Its external opening is homologous 

 with the anus of Rusconi, of Amphioxus and the Amphibians. 

 The ventral wall of the alimentary cavity is eventually derived 

 from cells formed in the yolk around the nuclei which are there 

 present. 



Since the important researches of Gegenbaur 1 upon the 

 meroblastic vertebrate eggs, it has been generally admitted that 

 the ovum of every vertebrate, however complicated may be its 

 apparent constitution, is nevertheless to be regarded as a simple 

 cell. This view is, indeed, opposed by His 2 and to a very 

 modified extent by Waldeyer 3 , and has recently been attacked 

 from an entirely new standpoint by Gotte 4 ; but, to my mind, 

 the objections of these authors do not upset the well founded 

 conclusions of previous observations. 



1 "Wirbelthiereier mit partieller Dottertheilung. " Miiller's Arch. 1861. 

 " Erste Anlage des Wirbelthierleibcs. 



3 Eierstock u. Ei. 



4 Entwicklungsgeschichte der Unkc. The important researches of Gotte on the 

 development of the ovum, though meriting the most careful attention, do not admit of 

 discussion in this place. 



