226 PRINCIPLES OF EMBRYOLOGY 



are confined to a smaller area than in amphibian eggs, although presum- 

 ably located in a similar position. 



The gastrulation process has been studied in some detail in the dog-fish, 

 Scyllium canicula, a selachian and therefore usually considered more primi- 

 tive than the sturgeons (Vanderbroek 1936). Its eggs, however, are more 

 like those of teleosts (higher bony fish). They are provided with a large 

 quantity of yolk, and the cleavage is partial, giving rise eventually to a 

 blastoderm. This gradually spreads over the whole surface of the yolk, 

 and as it does so invagination takes place along one sector of the circum- 

 ference, after which the neural folds appear in the same position. The 

 blastoderm is a fairly thick structure, and much of the endoderm lies from 

 the beginning in the depths of it beneath the surface. This is particularly 

 true of the extra-embryonic endoderm; some of the material which will 

 form the endoderm of the embryo proper is at first on the surface, lining 

 the position where the blastopore will appear. It is the first material to be 

 invaginated. The disposition of the other presumptive areas is shown in 

 Fig. 1 1.3. One point worthy of note is that the presumptive mesoderm 

 does not extend right round the blastoderm on to the ventral side, but is 

 confined to the dorsal region in the neighbourhood of the blastopore. 



2. Teleosts 



The eggs of teleosts are rather large, and contain considerable quantities 

 of yolk. Although there is extensive variation from species to species in 

 the relative mass of yolk and of living cytoplasm, the cleavage is always par- 

 tial and leads to the formation of a blastoderm. The structure of the egg at 

 this time is not altogether simple. At the animal pole there is a relatively 

 thick plate of well-defmed cells forming the blastoderm proper. Beneath 

 this is a cavity, and underneath that again and extending for some distance 

 around the blastoderm, the yolk is admixed with a fair quantity of 

 cytoplasm and contains scattered nuclei; this syncytium is known as the 

 periblast. Covering the whole surface of the egg is a thin cytoplasmic 

 membrane which shows the properties of low permeability, high contract- 

 ability, and lack of adhesiveness on its external face, which are also seen 

 in the external membrane of the amphibian egg. It may be referred to as 

 the 'coat', using the word employed by Holtfreter for the similar structure 

 in Ampliibia. Where it lies over the yolk outside the limits of the blasto- 

 derm, it is also knov^oi as the 'yolk gel membrane'. Its properties have 

 been particularly studied by Devillers (1948) and Trinkaus (1949). 



The blastoderm expands in area and after a time the sub-germinal 

 cavity below it becomes well marked (Fig. 11.2). The maximum depth 

 of the cavity which is roofed by the thimiest region in the blastoderm, 



