180 RICHARD ASSHETON. 



that the moment after the uterine wall comes to take the 

 place of the albumen layer as a support, then the greater part 

 of the expansion of the combined vesicles must be that part 

 furthest removed from the embryo, because it is here that the 

 uterus wall is infinitely less resistent, and with it will go that 

 part of the vesicle to which it is attached. 



On the Importance of the Albumen Layer and Zona 



radiata. 



Those who have followed my account of the development of 

 the blastodermic vesicle of rabbit between the eightieth hour 

 and the time of attachment of the vesicle to the walls of the 

 uterus at about the 170th hour, must have seen how important 

 a part the albumen layer plays in producing the form actually 

 assumed by the vesicle. I pointed out how that the thin cellular 

 wall of the blastodermic vesicle would be itself quite able to 

 withstand the hydrostatic pressure within ; and, further, that it 

 is not until the vesicle has attained such a size as to stretch the 

 walls of the uterus as to cause them to afford the support 

 necessary to prevent the bursting of the vesicle, that the albu- 

 men layer was lost. By this time the albumen layer has 

 become exceedingly thin, and the disappearance of it is brought 

 about by its rupture. Traces of it may be found curled up and 

 crumpled several days later. Once burst, its function, as far as 

 I can judge, is at an end. 



Now it has almost certainly at least one other very important 

 influence upon the development of the rabbit. By its presence 

 until the eighth day it absolutely prevents the cellular tissue 

 of the blastodermic vesicle from coming into close contact with 

 the cellular tissue of the uterus. The embryo up to tlie eighth 

 day is as free probably from the protoplasmic influence of the 

 mother, as is the egg of a bird after it has been covered with^a 

 thick calcareous shell. There is also, it will be remembered, 

 another coat to the ovum, the zona radiata, which is present 

 from the very first, and has a similar effect to that which the 

 albumen layer has, but being less thick its effect is more evanes- 

 cent. The two coats may be considered as one. 



