A SOUTH AMERICAN SPECIES OF PEHIPATUS. 225 



The organ described by Kennel as a placenta does not 

 appear till somewhat later. I shall not call it a placenta, 

 because it does not seem to me to bear any analogy to the 

 mammalian placenta. 



It seems to me that the best words to express the organs are 

 the embryonic and vesicular thickening corresponding to the 

 two parts of the placenta of Kennel (i. e. embryonic and uterine 

 placentas) . 



An early stage in its development is shown in fig. 16, which 

 represents part of the vesicle wall of a stage of the same age as 

 fig. 15 ; here it is seen to consist of a large mass of cells (pi.) 

 formed by the proliferation of the wall of the vesicle, which 

 under ordinary circumstances consists of a single row of cells 

 only. This is the vesicular thickening (= uterine part of the 

 placenta) as distinguished from the embryonic thickening (= the 

 embryonic placenta), which is merely the swollen part of the 

 vesicle wall from which the stalk of attachment arises (z.) . 



The vesicular thickening is found in its fullest development 

 rather towards the hinder end of the embyro, whereas the stalk 

 of attachment and its swollen base are at the head end of the 

 embryo. 



The histological structure of the vesicular thickening at this 

 stage is not remarkable ; it consists of a mass of nucleated 

 cells. The outlines of the cells are very apparent, and their 

 protoplasm is in parts much vacuolated. 



This stage corresponds in age to the embryo last described 

 (fig. 15), and is the earliest stage at which the vesicle swelling 

 is of any great size or importance. 



Fig. 17 shows a much further development of the vesicular 

 thickening ; owing to its large size only a small portion of the 

 vesicle wall (v. w.) is represented. 



The histological structure of the vesicular thickening has here 

 completely changed, all traces of cell walls have disappeared, 

 the nuclei are darker and more distinct, and the protoplasm 

 presents a very peculiar granular appearance which I have not 

 seen elsewhere. 



The vesicular thickening seems to be fused to the uterus wall 



