6 Mr. E. E. Prince on the Significance of the 



vious paper *, the globules in question seem to have no inti- 

 mate connexion with development, and are best regarded as 

 redundant and probably ancestral elements, still persisting, 

 but not immediately utilized by the germ. 



If this view be correct, that the yolk is a trophic appen- 

 dage, consibting in the later stages almost purely of inert 

 nutritive matter, that the germ is discoblastic and becomes a 

 discogastrula when the germinal cavity appears beneath it 

 (PI. II. fig. 10, ^.c), and hence that the invaginated rim 

 represents the primitive enteric involution, like the inflected 

 arc in Elasmobranchs and Amphibians, then the interpreta- 

 tion of the features presented by the Teleostean ovum becomes 

 greatly simplified. Balfour speaks of such a mass of unseg- 

 mented yolk as corresponding to the large cells of the vegetal 

 pole in a blastosphere ; and E. van Beneden similarly regarded 

 the deutoplasmic globe in a pelagic Teleostean ovum as a 

 large endodermic cell, with a constitution analogous to a fat- 

 cell tj a view shared by Hoflfman and others. But the Tele- 

 ostean germ never forms a blastosphere, with a more or less 

 centrally situated segmentation-cavity or blastocoel, in addi- 

 tion to the large subgerminal chamber, which is always 

 present at some stage. Van Bambeke alone amongst 

 observers really describes a blastocoel in the ^^^ of an osseous 

 fish ; but Oellacher, Kingsley and Conn, and other authors 

 regard such an intrablastodermic cavity as an artificial 

 product, and not a normal feature. The sub-blastodermic 

 cavity present in the Teleostean ovum (PI. 11. fig. 10, ^.c.) 

 must be the homologue not of the Amphibian and Selachian 

 segmentation-cavity, so-called, but of the enteric cavity, whose 

 external opening is the blastopore. The germ, thus separated 

 by a germinal cavity from the yolk, consists of two laraellse, 

 ectoderm and primitive endoderm, like a two-layered gastrula ; 

 the external layer or epiblast appears to be one cell in thick- 

 ness ; but the endoderm, or '' lower layer," consists of several 

 layers of cells (PI. II. fig. 10, g). From its mouth or blasto- 

 pore the yolk forms an enormous protruding mass, an exag- 

 geration of the yolk-plug which fills up the anus of Busconi 

 in Eana (PI. II fig. 10, y). 



The important feature in the Teleostean ^g2^ is not the fact 

 that the yolk is stored away at one pole of the &g^^ for the 

 ^g^ of the Amphibian or Cyclostome may be described as 

 simply the ovum of Aviphioxus with a large amount of trophic 

 matter stored away in its lower part, nor that the yolk-cells 



' '^ On the Presence of Oleaginous Spheres in the Yolk of Teleostean 

 Ova," Ann, & Mag, Nat. Hist, Aug. 1886, 



t Quart. Journ, Microsc. Sci, vol. xviii. 1878, p. 52. 



