412 THE EABBIT. 



cloacal dilatation, and separating the two structures from each 

 other behind the point of opening of the Wolffiaii duct, is a 

 septum of connective tissue, which is well seen in the figure, 

 where it is crossed by the reference line, TC. 



During the next three days this septum extends backwards, 

 its growth being effected by the union, in the median plane, of 

 two lateral ridges to form a median partition. This partition 

 divides the cloacal chamber into two separate portions, a dorsal 

 or rectal chamber, and a ventral or urine-genital chamber. The 

 proctodasal opening is established by this time, and the partition, 

 011 reaching the surface, divides this opening into separate 

 anal and urino-genital apertures, the partition itself forming the 

 perinaeum, or transverse septum between the two apertures. 



The anal opening lies on the posterior surface of the cloacal 

 papilla, almost in the angle between the papilla and the tail, so 

 that the cloacal papilla is from this time concerned with the 

 urino-genital organs alone. The cloacal, or genital papilla as it 

 may now be termed, elongates considerably, and the urino-genital 

 aperture is prolonged as a median groove along its dorsal or pos- 

 terior surface. From this stage, development differs in the two 

 sexes : in the male the two lips of the groove unite to form the 

 penial urethra, the papilla itself becoming the corpus spongiosum 

 of the penis. In the female the groove remains open, its borders 

 forming the lips of the vulva. 



DEVELOPMENT OF THE HEART AND BLOOD- 

 VESSELS. 



The general relations of the heart and its various cavities, 

 and of the great arterial and venous trunks, and the changes 

 which they undergo during development, are much the same in 

 the rabbit as in the chick, and it will not be necessary to describe 

 them in detail in this chapter. The changes in the heart itself, 

 and especially the mode of formation of the septa, by which the 

 several cavities are shut off one another, will require closer con- 

 sideration. 



Besides the vessels of the embryo itself, there are two extra- 

 embryonic vascular systems : (i) the vitelline, or yolk-sac cir- 

 culation, which is comparatively unimportant in the rabbit ; 

 and (ii) the allantoic or placeiital circulation, which is of the 



