COMPAEISON WITH EABBIT EMBRYOS. 485 



a change due possibly, as Professor His suggests, to backward 

 growth of the head fold of the amnion of the earlier stages, as 

 indicated by the dotted lines in Fig. 187 ; but more probably 

 caused, at any rate in part, by approximation and meeting of 

 the side folds of the amnion along the mid-dorsal line. 



His' diagrams (Figs. 186 to 188) undoubtedly give an intel- 

 ligible and consistent theory concerning the development of 

 the human embryo from the stage indicated by Heichert's ovum' 

 to that of the embryos B and SR, and it is greatly to be hoped 

 that opportunity may occur for testing their correctness by 

 actual observation. 



If the transition occurs in the way suggested by Professor 

 His, then both the amnion and the allantois of the human 

 embryo present features differing widely from those of the rabbit. 

 The amnion has no tail fold, which is almost the only part 

 developed in the rabbit ; while the allantois is, from the first, 

 continuous with the chorion. 



Even in the rabbit, however, an approach is made towards 

 the mode of development of the allantois supposed to occur in 

 the human embryo ; the mesoblast of the allantois in the rabbit 

 being, from the first, continuous with the mesoblast of the tail 

 fold of the amnion (Fig. 146), and very early fusing with the 

 chorion as well (Fig. 147). 



The precocious development of the allantois, which is one 

 of the most striking points about the human embryo, may be 

 connected with the precocious appearance of the vascular layer 

 of mesoblast lining the blastodermic vesicle ; and both features, 

 in so far as they are exceptional, may be regarded as examples of 

 the tendency to shortening or abbreviation of the processes of 

 development, which is so constantly encountered by the student 

 of embryology. 



The establishment of a vascular connection between the 

 embryo and the chorion, and so indirectly with the mother, is 

 the characteristic feature of mammalian development ; and it is 

 not surprising to find, in the most highly developed of all 

 Mammals, this feature thrown back to an earlier stage than that 

 at which it originally appeared ; and hurried on prematurely, 

 even at the expense, as it would seem, of the embryo itself, whose 

 development is unusually retarded. 



