234 PRINCIPLES OF EMBRYOLOGY 



It is however not completely absent, as Dalcq and Huang (1948) have 

 shown (p. 177). The teleost situation is, however, more comparable to 

 that in birds at the time of endoderm formation, as described by Lutz 

 (p. 183); there, again, the centre at which the embryo will form is not 

 sharply 'distinguished from the rest, and far reaching regulation is still 



possible. 



Devillers (195 1) has studied the orientation in which embryos develop 

 from portions of the trout blastoderm left in place on the egg or rotated 

 in various ways. His results indicate that, in spite of the power of regula- 

 tion which is spread throughout the whole blastoderm, the future embry- 

 onic region has akeady some shght degree of dominance both in the 

 blastula and still more the gastrula stages, and it tends to dictate the 

 orientation of the embryo. This again is very similar to the situation m 



birds. 11111 c 



The well-known evolutionary changes affecting the head skeleton ot 

 the fish have been discussed from the epigenetic point of view by 

 Devillers (1950). 



3. Reptiles 



Passing over the Amphibia, about which enough has already been said, 

 we come in our survey of vertebrate types to the reptiles. They have been 

 studied, recently, in the hght of our newer knowledge of other groups, 

 particularly by Peter (1938) and Pasteels (1936-7, i94o). The main points 

 which require notice concern the formation of the endoderm and the 

 invagination of the mesoderm. 



The eggs are large and full of yolk; cleavage is partial; and we meet 

 once more a blastodermal type of embryo. When the presumptive areas 

 are mapped, it becomes clear that the make-up of the blastoderm is 

 radically different from that of the teleosts. Instead of the presumptive 

 endoderm and mesoderm lying round the margin of the blastoderm, they 

 are located within it, the edge of the plate of cells being whoUy ectoder- 

 mal. There is considerable variation within the group in the mode of 

 endoderm formation. In the Algerian turtle Clemmys leprosa, Pasteels fmds 

 that the blastoderm at the end of cleavage is single layered. At a point, 

 which lies not at the edge of, but within, the blastodermic sheet, a groove 

 appears and cells of the single layer become pushed inwards to form the 

 endoderm, which thus owes its origin to a true invagination through a 

 blastopore. In other species, such as the chameleon and certain hzards, 

 the blastoderm never becomes single layered, and much, though perhaps 

 not all, of the endoderm is formed in place by a delamination similar to 

 that which was described above (p. I55) for the bird embryo. 



