ATTACHMENT OF MAMMALIAN EMBETO TO UTERUS. 181 



As regards the rabbit. I have shown in considerable detail 

 how many events are, or at least may be, explained by external 

 influences (such as albumen layer, hydrostatic pressure, pressure 

 of uterus, rupture of albumen layer) acting in conjunction with 

 a simple steady force or energy, the primary centre of cell mul- 

 tiplication. 



In doing so, it will be remembered that in connection with 

 the actual forms assumed, and phases passed through, in the 

 rabbit, the presence of the albumen layer, the size of the cavity 

 of the uterus, and even the shape of the walls of the uterus, had 

 important consequences ascribed to them. 



If my line of argument has been correct, the effects produced 

 by the albumen layer and the size and shape of the uterus must 

 be very different or absent altogether in forms in which these 

 conditions are different or absent. As regards the size of the 

 ovum itself, there is very little difference amongst mammals. 



Let us examine the case of a rat, — a rodent, and so not very 

 distantly removed genetically from a rabbit. And yet how 

 different is the form assumed in the earliest stages of develop- 

 ment ! Do the conditions differ from those in the rabbit, and 

 if so, how ? 



They differ in these respects : 



(i) There is no zona radiata or albumen layer. 



(ii) The diameter of the uterus is very much smaller, and the 

 lumen proportionately smaller still. 



(iii) The walls of the uterus are of a more uniform thickness. 



In the rabbit the zona radiata is so quickly covered by the 

 albumen layer that it cannot be said to have in itself much 

 effect as a support or protective coat. 



In the rat there is no zona radiata or albumen layer. The 

 ovum develops freely in the cavity of the uterus unprotected 

 by any coat. It would seem to pass very rapidly down the 

 Fallopian tube. Robinson found one early stage of segmenta- 

 tion; from this he figures the mesial section, which shows 

 parts only of twelve segments, so we may conclude that his 

 specimen was a fairly early stage of segmentation — comparable, 

 perhaps, to that of the rabbit of the forty-eighth hour. 



