FCETAL NUTRITION: THE PLACENTA 437 



Mouse. — Tke fertilised ova of the mouse reach the uterine 

 cavity on the third day, and segmentation is completed one day 

 later. The zona peUucida has by this time disappeared, and 

 fixation of the ovum to the uterus can be quickly , attained. 

 Each blastocyst comes to rest in an anti-mesometrial groove. 

 At first spherical, it becomes ovoid on the sixth day, with the 

 long axis perpendicular to the long axis of the uterus. One pole 

 is turned towards the mesometrium and is composed of several 

 layers of cells, while the opposite pole is single-layered. It is 

 nourished by the glandular secretion, and perhaps also by a 

 transudate, in which, however, leucocytes are not present. 



The connective tissue of the mucosa shows a thickening at 

 the point where a blastocyst rests. The epithelium degenerates 

 as the result of contact with the foetal ectoderm (Duval ^), or 

 of pressure by the prohferated connective tissue cells which 

 interferes with the nutrition of the epithehum (Burckhard ^). 

 More probably it is not mechanical, as the change begins first 

 at the mouth of the groove, i.e. at the point of first contact 

 with the ovum (Kolster ^). In the cells the chromatin clumps 

 on the inner surface of the nuclear membrane, the cell boundaries 

 disappear, and a symplasma is formed which later becomes 

 broken up into nuclear and cellular fragments. Fat globules, 

 which are present in the epithehum of the non-pregnant uterus, 

 are found in the detritus and also in the foetal ectoderm. 



With the destruction of the epithehum appears the first sign 

 of decidual formation. The connective tissue cells increase in 

 size and displace the glands ; the capillaries dilate irregularly, 

 and at places form sinuses. 



On the sixth day, the ectoplacental cone is formed by a pro- 

 hferation of the ectoderm at the mesometrial pole of the blasto- 

 cyst. It plugs the opening between the crypt and the lumen 

 of the uterus. At the same time the hps of the crypt are 

 gradually brought nearer to each other by the sweUing of the 

 tissues, and at the end of the seventh day they fuse and cover 



' Duval, "Le Placenta des Rongeurs," Joum. de I'Anat. et de la Phys., 

 1891. 



^ Burckhard, " Die Implantation des Eies der Maus in die UterinscMeim- 

 haut," Arch. f. mikr. Anat., vol. Ivii., 1901. 



' Kolster, "Zur Kenntnis der Embryotrophe beim Vorhandensein einer 

 Decidua Capsularis," Anat, Hefte, vol. xxii. 



