EAELT STAGES OF DEVELOPMENT OF THE EABBIT. 131 



Up to the moment of the beginning of the cavity there does 

 not appear to be any pressure upon the walls of the embryo by 

 zona radiata and albumen layer ; in fact, the embryo may often 

 be found to be slightly retracted from the zona radiata in the 

 fresh state. 



The diameter of the cavity within the zona radiata is, in the 

 unsegmented ovum, about 0*11 mm., and up to the time that 

 the cleft appears it has not greatly enlarged^ measuring only 

 about 0'12 mm. 



On the appearance of the cleft the cells of the outer layer 

 become pressed hard against the zona radiata and flattened, 

 but no great increase in diameter of the outer border of the 

 zona radiata is as yet recognisable, although the thickness of 

 the zona radiata is diminished, apparently being of a com- 

 pressible nature, and therefore becomes compressed between 

 the pressure from within the blastodermic vesicle and the 

 resistance afforded by the tough albumen layer from without. 

 At this time, being in the very firm lower part of the Fallopian 

 tube, the resistance afforded by the albumen layer is very 

 likely aided by the walls of the Fallopian tube itself. In this 

 condition the embryo usually passes into the uterus, although 

 sometimes specimens may be found in the uterus (but very 

 rarely) in which no cleft has as yet appeared. The embryos, I 

 think, pass rather suddenly through the last 4 to 6 mm. of the 

 Fallopian tube at some time between the 75th and 80th hours 

 after coition. 



No increase takes place in the thickness of the albuminous 

 layer after entering the uterus, but by the stretching of it 

 caused by the expansion of the blastodermic vesicle it rapidly 

 thins. The zona radiata thins so much as to be hardly per- 

 ceptible by the end of the fourth day. 



Van Beneden^s figures, 5, 8, 9, 10, 11, on pi. iv of his paper, 

 taken from optical sections, represent very well the appearances 

 presented by the embryos during these changes. 



As van Beneden's figures represent optical sections, it is, I 

 think, advisable to give a series of figures drawn from real sec- 

 tions, as none have hitherto been published. 



