ATTACHMENT OF MAMMALIAN EMBRYO TO UTEEUS. 185 



either zona pellucida or albumen-layer, and more probably 

 both. These are present until as late a period as in the 

 rabbit. There is no inversion in either case. The shape is 

 somewhat different in both cat and dog. The vesicles are 

 spherical until about the time that the resistance of the 

 uterine walls is encountered, after which time they assume a 

 much more oval shape. Whether this is due chiefly to the 

 effect of the walls of the uterus, or whether due in part to a 

 modification in intensity or of the extent of the trager area, it 

 is impossible to say without careful investigation. 



Amongst the Ungulates we have descriptions of the deer by 

 Bischoff, the sheep by Bonnet and Coste, the pig by Keibel. 



Although the literature on the point I am now discussing is 

 very scanty for this group of animals, we know two facts. 

 Firstly, that the vesicle, at first spherical, grows to an 

 enormous length — as much as 130 mm. in the pig (Keibel), 

 or the whole length of the uterus in cases where there is 

 only one embryo. Also we know there is no inversion. 



They are surrounded by a zona radiata or some other 

 investment, but it would seem to be very thin. Bischoff says 

 there is no albumen-layer in deer. 



As regards inversion, the Ungulates, as far as at present 

 known, behave like the Carnivora and rabbit. But after a 

 certain point the vesicle lengthens enormously. It seems 

 probable that this lengthening is due partly to the large 

 cavity in which the vesicles lie. All the Ungulates of which 

 we have any record are large. The lumen of the uterus 

 in the pig is enormous as compared with the lumen in the 

 uterus of a rabbit. 



Also there seems to be evidence to show that the zona 

 radiata or albumen layer is thinner, and offers therefore less 

 resistance, and accordingly has less effect in resisting the 

 pressure of the walls of the uterus, which, it must be remem- 

 bered, actually lie in contact with each other. 



We must also suppose that although the pressure is sufficient 

 to prevent the vesicle from retaining an approximately spherical 

 form, it is not as yet sufficient to cause a fixation of the em- 



