430 IMPLANTATION OF THE OVUM IN 



pig. This has been extensively studied, but we owe to Bischoff ('52, 70), Reichert 

 ('62), Hensen (76, '83), Selenka ('84), Duval ('89-'91), and particularly to Graf Spee 

 ('83, '96, :01) most of our knowledge of its early development. Bischoff first noted 

 the passage of the ovum either through the uterine epithelium or into the mouth of 

 a gland which after closing up shut off the blastocyst from the uterine lumen. Selenka 

 also describes and figures the blastocyst as entering a gland-mouth. Hensen, fol- 

 lowed by v. Spee ('83), showed that the trophoblast cells at the anti-embryonal pole 

 (Gegenpol) gave off protoplasmic processes which perforated the zona and possibly 

 aided in the attachment of the ovum to the uterine epithelium. V. Spee in his pre- 

 liminary paper ('96) and in the full paper (:01) gives a detailed description and many 

 figures of the process of implantation. 



At the time of entrance into the uterus, the ovum of the guinea-pig is extremely 

 small, having a diameter of 0.1 millimetre less 0.08 millimetre, the thickness of the zona, 

 and is in a late morula or beginning blastula stage. Surrounded by the zona, it passes 

 into the ventral portion of the slit-like uterine lumen during the seventh day. It is only 

 amatter of two to eight hours before the ovum has passed through the uterine epithe- 

 lium and become encapsulated in the subepithelial connective tissue. The tropho- 

 blast cells of the blastocyst where it comes into contact with the uterine epithelium 

 cause the destruction of the epithelial cells and the whole blastocyst passes through 

 into the connective tissue. The adjacent connective-tissue cells become softened 

 and fused into a finely fibrillated granular mass with free nuclei, the symplasma of 

 v. Spee. This symplasma extends irregularly into the connective-tissue stroma and 

 later becomes vacuolated and thin, leaving a lymph-space j)r extra-uterine decidual 

 chamber in which the blastocyst will complete its development. 



The symplasma of the guinea-pig has certain resemblances to the fixation-mass 

 of Spermophilus in its general syncytial character, its fibrillae, and the extension of 

 its processes into the adjacent stroma (compare Figs. 37, 38, PI. XXXI, of this paper 

 with v. Spee's, :01, Figs. 13a, 136, Taf. X). However, the fixation-mass of Sper- 

 mophilus differs in showing no vacuolation and in being derived from the trophoblast 

 instead of from the connective tissue. 



Regarding the early development of the Sciuromophi little work has been done 

 except by Fleischmann ('92, '93), who has described certain stages in the development 

 of Sciurus vulgaris and the badjing, a Javanese squirrel, and has given an account 

 of the placentation of Spermophilus citillus. This is the only paper known to me 

 on the development of Spermophilus. I can in the main confirm Fleischmann's results 

 in the later stages, but he did not have young enough material at his disposal to give 

 any_description of the preplacental conditions. 



