STUDIES IN MAMMALIAN EMBRYOLOGY. 311 
lates in embryonic tissue from the earliest down to the latest 
stages of pregnancy ? 
With these questions we will have to deal in the following 
chapter. 
Il. The Histological Modifications in the Uterine 
Tissue during Pregnancy. 
In examining transverse sections of a uterus of the hedge- 
hog in the earliest stages of pregnancy, the remarkable dif- 
ference cannot escape detection which obtains between the 
four to eight spots where the blastocysts lie, as compared to 
the intervening regions. Even though there may be as yet no 
externally visible sign that the uterus is indeed pregnant, save 
a very faint swelling. ‘This swelling is in some cases hardly 
conspicuous, and it is certainly less marked than other local 
swellings that often accompany the period of involution of the 
uterus after parturition. These latter stages are particularly 
noticed in August and September, and are easily mistaken for 
early stages of pregnancy—a mistake which it is all the more 
important to notice as sometimes these very uteri are found to 
be pregnant for a second time in the same season. Careful 
examination is thus necessary in every particular case. 
The difference above referred to between the spots where the 
blastocysts lie and the intervening regions is diagrammatically 
represented in figs. 1 and 2 of Pl. XV. Fig. 1 shows a trans- 
verse section of the uterus, intermediate between two swellings, 
in which the blastocysts will become embedded. Here the 
lumen is compressed, and the utricular glands, coiled in their 
deeper portions close’to the blind ends, are in so far unequally 
distributed over the surface that the portion of the mucosa 
adjoining the mesometrium has much shorter glands, and is 
at the same time much less high, than the mucosa of the three 
other sides. The same difference is noticed in the uterus of 
young female hedgehogs that have evidently never been preg- 
nant. In these the mucosa is thinnest in the mesometrical 
VOL, XXX, PART 3,—NEW SER, a 
