346 W. L. SCLATER. 



of the uterine epithelium is not seen very well in the longitu- 

 dinal section (fig. 1), but it is better seen in fig. 5. 



At either end of the embryo is seen a mass of pigment (pg.), 

 in the form of black, star-shaped masses, which seem to block 

 up the lumen of the uterus ; around and near them are certain 

 darkly-staining irregular masses of protoplasm (sc), in which no 

 structure whatever can be distinguished. Between the em- 

 bryos is seen the very curious vacuolated tissue described by 

 Kennel, of which it is very difficult to understand the meaning. 



This tissue (v. c.) consists of at first a more irregular, after- 

 wards of a more regular mass of vacuoles, separated from one 

 another by thin lines of protoplasm which stain but slightly ; 

 along the outer border of these vacuoles there is a line of 

 nuclei (n.) which are considerably larger than those of the 

 ordinary uterine epithelium. 



The appearance of this vacuolated tissue in transverse 

 section is very well shown by Kennel (Part 1, PI. VII, 

 fig. 42). 



The lines of protoplasm separating the vacuoles run straight 

 from the central point of the section, where occasionally some 

 remains of the lumen of the uterus can be seen, to the circum- 

 ference where the vacuolated tissue comes in contact with the 

 uterine epithelium. Just at this point, but in the vacuolated 

 tissue, are found the numerous large nuclei generally arranged 

 in a single row (n. in fig. 1). 



This vacuolated tissue is never found near the embryo; 

 where the embryo is present the vacuolated tissue is entirely 

 replaced by the uterine epithelium, as is seen in fig. 1. 



This vacuolated tissue is probably simply modified uterine 

 epithelium, though how the change has been effected I am 

 unable to suggest. 



The explanation of this curious histological structure in the 

 uterus of P. imthurni seems to me connected with the 

 entire absence of yolk, and the small size of the ovum of this 

 form. The difference in size between the youngest embryo (a 

 sphere measuring "04 mm. in diameter) and the fully formed 

 one, which is often an inch long, is enormous, and there must 



