240 RICHARD ASSHETON. 



any such process as a growth of one group of cells round 

 another group in the rabbit. 



In 1894 a renewed study of this interesting stage was under- 

 taken by myself (1). I examined a very large number of 

 segmenting ova, both fresh, and by actual section. I was able 

 to confirm Heape's account of the irregularity of the segmenta- 

 tion process, and quite failed to find any decisive evidence of 

 the metagastrula stage at all. In fact^ the balance of evidence 

 was all against it, and I came to the conclusion that van 

 Beneden's description was not supported by a study of the egg 

 by means of sections. 



When, therefore, I read Hubrecht's paper on Tupaia and 

 Duval's on bats^ and made my own observations upon the sheep, 

 I was not a little puzzled to know what to think of the rabbit. 



To accept van Beneden's account, as Duval proposes to do, 

 leads us again into all the old difficulties regarding the fate of 

 the inner mass, and, as explained above, it is, I think, impossible 

 to admit Duval's method of escape from them. 



Let it be supposed, on the other hand, that we admit van 

 Beneden's view as far as the formation of the metagastrula 

 is concerned : is it not possible that this stage is succeeded 

 by a stage in which the outer layer becomes doubled, as in 

 my fig. 11 of the sheep, and that the cavity of the blastodermic 

 vesicle appears really between the cells of this doubled outer 

 layer ? For instance, van Beneden's (9) pi. iv, figs. 4, II, is very 

 much like my figure of the sheep (fig. 9). The outer layer is 

 distinctly thicker at one side than the other. May not this be 

 the commencement of a reduplication of the outer layer? 



In the absence of any diff'erence in colour or other recog- 

 nisable characteristics of the two groups of cells it is not 

 possible to affirm that this is the course of events, but it is 

 equally impossible to deny it. 



I have again gone over the same ground in the rabbit with 

 new material, and have treated it as far as possible in the 

 same way that I treated the sheep embryos. 



On examination of these embryos I am forced to come to 

 the same conclusion to which I came three years ago. In the 



