236 PRINCIPLES OF EMBRYOLOGY 



makes them seem likely to be difficult experimental material, and no one 

 has yet had the opportunity or the boldness to tackle them. 



For the sake of continuity, it is worth while repeating two ponits about 

 the avian embryo in relation to what has just been said about the reptilian. 

 In the first place, endoderm formation is probably by delammation, as m 

 turtles, not by invagination from a definite blastopore. Secondly, while 

 mesoderm invagination is beginning, streaming movements take place 

 from the posterior towards the anterior; thus instead of the mesoderm 

 beincr formed from a circumscribed blastopore which leads m to a long 

 forwlrdly extending chorda-meso dermal canal, it originates from an 

 elongated primitive streak, from the anterior end of which there juts 

 forward only a short extension of invaginated material, namely the head 

 process. 



4. Mammals 



The major characteristic of the mammals (except for the most priniitive 

 group) is the adaptation of their embryos to intra-uterine life. Evolution 

 has in fact, been particularly active in the bringing about of changes m the 

 early stages of development, and the group as a whole shows a very wide 

 range of different conditions, which we shall only be able to sketch m 

 very broad outline. 



In the lowest group, the prototherian mammals or monotremes, such 

 as Echidna, the egg is still reptilian in general configuration, cleavage is 

 partial, and a blastoderm is formed. Flynn and Hill (i939, 1942) fmd that 

 endoderm formation takes place mainly by delammation, though there 

 is also some movement of isolated cells out of the upper layer mto the 



deeper one. 1 \ 1 r r^-V, 



In the higher groups (marsupials and true mammals), the torm ot the 

 ep-g is completely different (Review: Nelsen 1953). It is small, and con- 

 tains little if any yolk. Cleavage is total. Endoderm and mesoderm forma- 

 tion, however, occur by processes wHch are clearly modifications of those 

 seen in the reptilian ancestors. The cleavage gives rise at first to a rather sohd 

 mass of cells, but a cavity soon appears amongst them This is excentri- 

 cally placed, so that the embryo assumes the shape of a hoUow sphere, or 

 blastocyst, to the imier face of which a thicker cluster of ceUs adheres 

 at one place (Fig. 11.7). This is the 'inner cell mass', and from it the mam 

 embryo will be formed. The remainder of the sphere is extra-embryonic 

 'trophoblast', concerned with anchoring the embryo to the wall ot the 

 uterus; it is probably to be compared with the outermost parts ot the 

 reptihan and bird blastoderms, the area opaca. From the mner cell mass, 

 the endoderm probably forms by simple delamination, though little is 



