DEVELOPMENT OF ELASMOBRANCH FISHES. 55 



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^ 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' and to a 

 very modified extent by Waldeyer^ and has recently been 

 attacked from an entirely new standpoint by Gotte"; but, to 

 my mind, the objections of these authors do not upset the 

 well founded conclusions of previous observations. 



As soon as the fact is recognised that both meroblastic 

 and holoblastic eggs have the same fundamental constitution, 

 the admission follows, naturally, though not necessarily, that 

 the eggs belonging to these two classes differ solely in degree, 

 not only as regards their constitution, but also as regards the 

 manner in w^hich they become respectively converted into the 

 embryo. As might have been anticipated, this view has gained 

 a wide acceptance. 



Amongst the observations, which have given a strong 

 objective support to this view, may be mentioned those of 

 Professor Lankester upon the development of Cephalopoda^ 

 and of Dr Gotte^ upon the development of the Hen's egg. In 

 Loligo Professor Lankester showed that there appeared, in 



1 Wirbelthiereier mit partieller Dottertheilung. MuUer^$ Arch. 1861. 



2 Erste Anlage des Wirbelthierleibes. 



3 Eierstock u. Ei. 



^ EnUcicklungsgeschichte der Unke. 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. 



5 Annals and Magaz. of Natural History, Vol. xi. 1873, p. 81. 



6 Archiv f. Mikr. Anat. Vol. x. 



