436 Blastodermic Layers in the Rat and Mouse. [Dec. 21, 



cesses which are continued over the surface of the placenta as far as 

 the central place of insertion of the allantois. 



The arrangement of the fully formed membranes is now easily 

 seen : the wall of the blastodermic vesicle is the external one, and is 

 continuous with the margins of the placenta ; it becomes much 

 stronger in the older stages. Inside this membrane is one formed of 

 hypoblast and vascular mesoblast and which was continuous with the 

 alimentary canal, while inside the interamniotic space is the stalk of 

 the allantois. 



I do not at present enter into a further description of the general 

 phenomena of foetal development, because after the closure of the 

 neur-amniotic cavity and the establishment of the relation of the 

 parts already described, the subsequent phenomena are essentially 

 similar to those of other animals. 



As to the cause of the peculiar form of development of the blasto- 

 dermic layers in some of the rodents to which reference is made in 

 this communication, it would be premature to speculate until its con- 

 ditions have been more fully ascertained ; but I would remark that 

 this peculiarity appears to stand in some close and constant relation 

 to the very early, rapid, and voluminous formation of the solid mass 

 of decidua within which the ovum is from the first enclosed in all 

 those animals in which the so-called inversion of the layers has been 

 observed. 



Since the observations above recorded were made, three short 

 papers bearing upon the subject have appeared in Germany during 

 the month of November, one by Professor Hensen, of Kiel,* dealing 

 with the guinea-pig, another by Professor Kupffer, of Munich, f 

 on the field mouse, and a third by Professor Selenka, of Erlangen, J on 

 the white mouse, all of which were unknown to me until some time 

 after I had arrived at the conclusions stated in the preceding pages. 

 Without entering into any detailed criticism of the contents of these 

 papers, I may state that the main difference between Professor 

 Kupffer and myself has reference to the early condition of the 

 epiblast, which is solid in the rat, but which he describes as forming 

 in the field mouse a single involuted layer. 



In conclusion I have to express my warmest acknowledgments to 

 Dr. Thomson, who has gone over all these observations with me, and 

 to whom I am indebted for many suggestions in my description of 

 this work. 



The figures are diagrammatic. 



* " Verhandl. des Physiol- Vereins in Kiel." Sitzung vom 2 Nov., 1882. 

 f " Sitz. Ber. d. k. B. Akad. der Wiss." 4 Nov., 1882. 

 X " Biolog. Centralblatt." 15 Nov., 1882. P. 550. 



