SPERMOPHILUS TRIDECEMLINEATUS MITCH. 425 



and these changes gradually extend into the outer layer of connective tissue. The 

 intercellular lymph-spaces and budding capillaries extend close up to the basement 

 membrane. The growth of the mucosa is most marked in the lateral and antimesome- 

 trial portions of the uterine tube. In the placental chamber and the intermediate 

 portion the gland-ducts become dilated; while in the lower and ventral portions sur- 

 rounding the fixation-chamber both glands and ducts are lost to a large extent, and 

 the connective-tissue stroma of the mucosa is increased in volume by the rapid cell- 

 proliferation. 



What determines the point of attachment of the ovum to the uterine wall ? Is 

 it accidental or predetermined ? These questions cannot yet be fully answered. In 

 the guinea-pig Spee ('83) has figured protoplasmic processes extending from the tro- 

 phoblast cells at the fixation-pole (Gegenpol, Spee), which may attach themselves 

 to the uterine epithelium. Born, shortly before his death, suggested the theory 

 that, during the time the ovum remains in the oviduct, the corpus luteum acts as 

 a gland whose internal secretion is carried by the blood-vessels to the uterine mucosa 

 and causes the cell-growth and prepares for the attachment of the ovum. The experi- 

 ments by Fraenkel und Cohn ( : 02) in this direction are suggestive as far as they have 

 been carried. 



In Spermophilus, I believe, the power of attachment resides very largely in the 

 blastocyst. It will be remembered that the path through the fixation-chamber along 

 the length of the uterine horn is a tortuous one with many lateral projections on either 

 side (PL XXX, Fig. 15), and at intervals grooves or depressions in the floor (Figs. 

 5, 11, 12, etc.). The blastocyst is probably aided by a peristaltic movement in mak- 

 ing its way along the uterus. It is most certainly undergoing profound structural 

 differentiation during this journey; and it seems quite reasonable that the fixation- 

 mass may be acquiring chemotactic properties which when sufficiently developed 

 will cause the blastocyst to come into contact with the epithelial wall at once. Any 

 part of the wall of the fixation-chamber may be the focal point, the sides as well as 

 floor (compare Figs. 4, 6, and 27 with Figs. 7, 8, and 30). In several cases where 

 the fixation-chamber was narrow the blastocyst made a double attachment, setting 

 up the same series of changes in the epithelium of both sides at the points of contact. 



B. Blastocyst.— It is the trophoblast or extragerminal ectoderm that is of most 

 interest at this time. This outer cell layer of the blastocyst, which at first envelops 

 the inner cell-mass but is independent of it, undergoes many changes in the various 

 mammals which have been studied. In the present paper one more is added to the 

 list. 



In Figures 5, 12, 13 (PI. XXX) the blastocyst is shown in its free condition 



