MORPHOLOGY OF THE HUMAN URINOGENITAL TRACT. 333 



"known as the cloacal membrane, reaching from the body end of 

 the future navel to the limit between entodermal cloaca and 

 end-gut. This boundary has at first no mesoblast, is related in 

 part to the primitive streak, i.e., is developed from its posterior 

 ■end, and is probably due to a drawing out of the blastopore. 



A few words may be added on this point so as to make the 

 matter quite clear. In the development of the amphioxus we 

 have first a cell proliferation which leads to the formation of a 

 hollow sphere — the blastula (fig. 2). This invaginates and 

 gives a double-layered structure (ectoderm and endoderm) — 

 the gastrula — with an aperture of invagination — the blastopore 

 (fig. 3). This gastrula stage is supposed to hold good for verte- 

 brates in general (Haeckel and others). 



In the germinal area of the rabbit we get an axial thickening 

 ■of the ectoderm — the primitive streak — which develops lateral 

 thickenings and also an aperture at the anterior end, the neur- 

 enteric canal which forms a communication between endoderm 

 and ectoderm. 



In the early human embryo (2 mm.), Graf Spec has found 

 the canalis neicrentericus which unites the endoderm of the um- 

 bilical vesicle with the ectoderm, and represents the blastopore 

 of the gastrula stage (fig. 4). Keibel in his sections of embryo 

 E B (caudal end ; greatest length, 3 mm,), figures the cloacal 

 membrane and remains of the primitive streak on it, so that it 

 is probable that the cloacal membrane represents or is derived, 

 in part at any rate, from the posterior end of the primitive 

 ■streak, along with a drawing out and union of the edges 

 of the blastopore (which in itself lies in the anterior part 

 of the streak), and this would explain the want of meso- 

 blast at first. The cloacal membrane extends up as far as the 

 root of the future navel and back to the front of the future 

 anus, as one of the malformations I have to consider later on 

 tends to demonstrate. The development of the cloacal mem- 

 brane is of great interest, as its patency in the new-born foetus 

 gives rise to the rare condition known as extroversion of the 

 bladder, and explains cases of split pelvis, split clitoris, etc., as 

 I shall indicate further on (v. paper following, p. 376). 



Before detailing the further changes, I wish to remark on 

 the unsatisfactorv nature of the nomenclature in relation to 



