216 RIOHAED ASSHETON. 



embryo was lyiug uaked and rather crumpled in the cavity of 

 the uterus. In the embryonic mass an oval body is sharply 

 marked out^ which probably gives rise to the epiblast of the 

 later stages. I have, unfortunately, not been able^ to obtain 

 the stages between my fig. 21 and Bonnet's earliest stage, in 

 which there was no trace of the trophoblast layer over the 

 embryonic epiblast. I cannot see that there can be any reason- 

 able doubt as to the fate of the mass E. The more loosely 

 arranged masses lying at the sides and lining the inner surface 

 of the epiblastic knob can be recognised as the hypoblast. 



Fig. 17 probably represents a stage only very little older 

 than the former embryo (fig. 16), for both are from the same 

 sheep ; yet the change in appearance of the inner mass is very 

 marked. One cannot help thinking that the very decided 

 change is due in some way to the loss of the zona radiata, 

 and that in all probability some such delimitation as seen in 

 fig. 17 really already exists in fig. 16, but that, owing to the 

 greater tension under which the whole structure must be, tlie 

 delimitation is masked for the time being. 



In my account of the development of the pig" I have laid some 

 stress upon the sudden splitting up of the embryonic epiblast 

 and hypoblast into distinct layers, and I noted the coincidence 

 of this phenomenon with the loss of the zona radiata. In this 

 respect the sheep and pig correspond. 



If it is possible, as I believe it to be, to identify the oval 

 knob of fig. 17 with the lighter cells of fig. 15, and again these 

 with the extremely plainly defined lighter cells of fig. 11, we 

 have here a most striking developmental history. The hypoblast 



* Since writing the above I have obtained a twin specimen of the sheep, 

 i.e. a blastocyst with two embryonic masses, each apparently normal, with 

 the exception of being only half the usual size. In one of these the tropho- 

 blast has disappeared over the centre of the epiblast, which at this spot shows 

 Indications of a pitting in which recalls the condition of otiicr mammals 

 (Tupaia, Sus) accompanying the rupture of the trophoblast over the epiblastic 

 knob. A description of this specimen is given in the ' Journal of Anatomy 

 and Physiology,' vol. xxxii, April, 1898. 



^ This account is already in print, and will be published in the next number 

 of this Journal. 



