238 PRINCIPLES OF EMBRYOLOGY 



suggests that there is less longitudinal movement up and down the streak 



than in birds. 1 • r 1 



The first few steps have been taken towards a causal analysis ot early 

 mammalian development. Dalcq (1951^, 1952), and several of his pupils 

 (cf Jones-Seaton 1950) are engaged in a detailed study of the differential 

 staining and other properties of the cytoplasm of the oocyte and early 

 developmental stages of the eggs of a number of species. Their most 

 important results so far are, perhaps, the demonstration that there is some 

 degree of bilateraHty in the structure of the oocyte and unfertihsed egg; 

 and that the distinction between the embryonic and extra-embryomc 

 regions arises gradually, and is not fixed at, for instance, the first cleavage 

 division as has sometimes been suggested (Fig. 1 1. 8). . r^u- ■ 



The latter conclusion accords well with experimental work. This is, 

 of course, technically very difficuk in mammals. It is first necessary to 

 operate on the young stages, to remove the eggs from the mother, then 

 to divest them of the tough jelly which surrounds them or to operate 

 through it, and fmally to return them to some situation m which they will 

 continue their development. Rodent eggs, which are easy to obtain, 

 seem very sensitive to the removal of their membranes, and are ditticult 

 to keep alive in any situation except the uterus, although they can be re- 

 transplanted back into another uterus with fair success (cf Nicholas 1947, 

 WiUett 1953). The rabbit egg is in general tougher; the embryonic area 

 at the primitive streak stage can be cultured in vitro for a time long 

 enough for a fair amount of differentiation to occur. It is worth notmg 

 (cf p. 231) that the older the embryo, the better it stands the conditions 

 of artificial cultivation (Waddingtoni937). ...11 j 



A httle work has been done on the early cleavage stages. Nicholas and 

 Hall (1942) succeeded in getting some development (up to just before the 

 time the embryo makes its appearance) of one of the first two blastomeres, 

 the other having been destroyed or removed. Pincus (1936) got rather 

 similar resuks with the rabbk. Recently Seidel (1952^ had been much 

 more successful. One of the first two blastomeres of a rabbit egg was kiUed 

 by pricking with a glass needle, without removing the jeUy layer, ihe 

 eg/was then mjected mto the Fallopian tube of another rabbit at a suit- 

 able stage in the reproductive cycle, and fully norm.al young were born. 

 The evidence that one blastomere was really killed seems quite con vmcmg, 

 and the operated egg differed genetically (in colour) from the host, so that 

 there is no doubt that the young animal was derived from it and not 

 from an unkijured egg of the foster-mother. Only two such ammals have 

 so far been described; one was fully normal, but the other showed defects 

 on one side, rather similar to those which are found m newts eggs from 



