1905.] PLACENTA OF THE SPINY MOUSE. 287 



large efferent vessels which presumably debouch into two maternal 

 veins (MV). 



These large efferent maternal vessels are more peripheral in 

 position than the afferent courses. 



The fcetal blood-supply is less easily followed. The allantoic 

 arteries, on reaching the surface of the placenta, radiate and 

 subdivide and give off branches which pass into the placenta at 

 intervals over the more central part of the surface. These pass 

 (as regards the main stream) straight through the foetal half of 

 the placenta, but give off at frequent intervals small capillaries, 

 which take a more sinuous and radial course, anastomosing, forming- 

 network, and collecting together again into veins, which I believe 

 to run parallel with the arteries of the villus. 



A considerable amount of connective tissue accompanies these 

 villi for the first part of their ingress into the placenta. 



Comparison urith other Placentas. 



The placenta is clearly of the type which I have described 

 elsewhere as cumulate as contrasted with the plicate type 

 characteristic of such groups as the Ungulates. 



The general shape, the arrangement of the membranes and 

 their character, resemble, according to the interpretation placed 

 upon them above, the condition that pertains in Mns musculus, 

 and, rather less closely, that of Mtis decitmanus. 



Jenkinson has recently given an account of the development of 

 the placenta of the Mouse, which differs in several important 

 particulars from that of Duval, which, until that time, had been 

 generally accepted. 



Jenkinson agrees with Daval as regards the general arrangement 

 of membranes and in the main features of the development. 



He differs, however, in respect to the vascular system, and my 

 account given above is in complete agreement with Jenkinson. 

 I see also nothing to suggest the appearance of an ingrowth of 

 the trophoblast into the maternal blood-vessels, and growth along 

 the inside as described by Duval ("plasmode endovasculaire "). 



Another point of disagreement is with reference to the 

 glycogenous tissue. Duval takes but little notice of this and does 

 not seem to have found the maternal glycogenous mass, which, 

 according to Jenkinson, degenerates, and the space occupied by it 

 becomes subsequently filled with a second glycogenous tissue 

 which is of foetal (trophoblastic) origin. 



This is a matter of very considerable interest. It is not to be 

 expected that an isolated case in an allied genus can afford any 

 conclusive evidence. 



Unfortunately the method of preservation (corrosive sublimate) 

 is not suitable to the study of glycogen. 



Tested with iodine I find no trace of glycogen, but there are 

 certain spaces in the trophoblast cells alluded to in the foregoing 



