SPERMOPHILUS TRIDECEMLINEATUS MITCH. 431 



From the above it will be seen that Spermophilus differs from the other rodents 

 and, further, from any other mammal yet described in the temporary fixation-mass. 

 It agrees with the rabbit in using the whole uterine cavity as the true decidual chamber, 

 in having a corresponding site for placental attachment, and in the loss of Rauber's 

 layer from the germinal area. In the Myomorphi only the ventral portion of the 

 uterine cavity is used; and in the Hystricomorphi the decidual chamber is outside 

 the uterine cavity altogether. Spermophilus resembles the guinea-pig in many 

 details of the perforation of the epithelium; but, while this process is permanent 

 in the guinea-pig, it is only temporary in Spermophilus. Spermophilus agrees with 

 the rabbit and differs from the mouse and guinea-pig in the absence of the so-called 

 inversion of the germinal layers. 



Regarding the other discoidal placental mammals, Insectivora, Cheiroptera, 

 Primates, a number of very important contributions to our knowledge of the pre- 

 placental or early stages of development in each of these groups have been published 

 during the last few years. In the Insectivora the investigations of Heape ('83-'86), 

 Strahl ('89), and Vernhout ('94) on Talpa, and Hubrecht ('89, '90, '95, '96) on Erina- 

 ceus, Sorex, Tupaja, and Galeopithecus, show that in this group there are also pro- 

 nounced variations in the size of the ovum, the site and manner of implantation, and 

 the beginning of the placental formation. As in the rodents, so here, the trophoblast 

 plays the active, the uterine epithelium the passive role. The Cheiroptera, as repre- 

 sented by several European species studied by Van Beneden ('88), Duval ('95-'96), 

 Frommel ('88), and Nolf ('96), and in Pteropus, studied by Goehre ('92), show in like 

 manner generic differences in the details of implantation. Hubrecht ('96) has given 

 us further important details along these lines for Tarsius, and Selenka ('92, '99, : 00) 

 and Strahl ('99) for apes. 



Selenka has shown in some of the apes that implantation takes place by means 

 of the attachment of the trophoblast to the ventral wall of the uterus, and that later 

 the trophoblast on the opposite side of the blastocyst develops a secondary placental 

 attachment to the dorsal wall. 



As is to be expected, there is not perfect harmony in all the details of the results 

 obtained, nor in the hypotheses based on these results. And when we consider how 

 few genera and fewer families have been studied in these orders, one sees the great 

 need of further investigations in the early stages of development extended over a 

 wide range of genera in each of these great groups. 



These investigations have yielded important results, and have given a better 

 and clearer understanding of mammalian development in general. They have done 

 away with many old and faulty theories and given a basis for theories probably more 



