428 IMPLANTATION OF THE OVUM IN 



the germinal area come into contact with the hitherto unaltered epithelium of the 

 placental chamber, and the beginnings of the true placenta are inaugurated. The 

 many interesting details connected with the placenta will be described and illustrated 

 at another time. 



B. Blastocyst. — The blastocyst is now a large, oval vesicle whose walls are 

 coming into closer relation with the epithelium of the fixation-chamber. The 

 trophoblast cells in many places are flattened into very large and extremely thin 

 scales (PI. XXXI, Figs. 32, 33). The deeply cupped fixation-mass now undergoes 

 marked changes. The root-like processes gradually atrophy and disappear, the cup 

 becomes more and more shallow, the nuclei decrease in number, and finally, with 

 the loss of the processes, the fixation-mass separates from the connective tissue; and 

 once more for a short time the blastocyst is free in the uterine cavity. The attach- 

 ment at the embryonal pole to the borders of the placental chamber soon follows. 



5. Comparison of the Implantation in Spermophilus and that in other Mammals. — It 

 is desirable first of all to consider the fixation of Spermophilus in comparison with 

 that of other rodents. This order is a primitive one, very widely distributed, and 

 contains a larger number of families, genera, and species than any other order of 

 mammals. The ease with which certain species may be kept in captivity, the frequency 

 with which they breed, and the large number in a brood, have resulted in a few species 

 becoming the subjects of a long and extensive series of investigations upon the stages 

 preceding and during placentation. It will be impossible, here, to give any extensive 

 review of the literature, and I shall confine myself to calling attention to certain par- 

 ticular points and to some of the more important monographs. Sclater states that 

 the order Rodentia contains 21 families, 159 genera, and 1400 species. The develop- 

 ment of scarcely a dozen species, representing four families, has been studied in 

 any great detail. Much more work must be done on this order before we can apply 

 embryological facts in any other than a tentative manner to the problems of classi- 

 fication. It will be convenient, in our discussion, to use the old grouping of the rodents 

 into Sciuromorphi, Myomorphi, Hystricomorphi, and Lagomorphi. 



The early development of the Myomorphi has been carefully studied in several 

 species of mouse and rat by Kupffer ('82), Selenka ('83), Duval ('89-91), Cristiani 

 ('92), Robinson ('92), Sobotta ('95, :01), Jenkinson (:00), D'Erchia (:01), Burckhard 

 (:01), and many others. 



In the mouse and rat Selenka ('83), Duval ('89-91), Sobotta (:01), and Burck- 

 hard (:01) have most carefully described and figured the first relationship of the blas- 

 tocyst to the uterine epithelium. The young blastocyst sinks into the ventral portion 

 of the slit-like uterine lumen, but does not reach the floor by the end of the fourth 



