382 DR D. BERRY HART. 



anus (fig. 18, preceding paper, p. 364). In the adult, there- 

 fore, the cloacal membrane is represented by the mesial region 

 from navel back to perineal body — the anterior boundary of 

 bladder, pubes, urethra, and vestibule. We have now to 

 discuss the significance of the neurenteric canal and of the 

 cloacal membrane. Kowalewsky was the first to shew that 

 in the echinodermata a free swimming larval form was formed 

 by an invagination of ectoderm, and that we thus got a two- 

 layered condition — ectoderm and entoderm — with the blastopore 

 of Lankester as evidence of invagination. This form is known 

 as the gastrula, and the process I have described as gastrula- 

 tion. Haeckel's gastrea theory — viz., that in higher animals 

 we would have the same process — has been amply confirmed, 

 and the neurenteric canal in the human embryo may be regarded 

 as a variety of gastrulation. Selenka has found conditions 

 exactly similar to those of Graf Spee's embryo in the Bornean 

 gibbons (figs. 2, 3, and 4). 



The cloacal membrane is practically derived from the blasto- 

 pore or neurenteric canal i.e., the dorsal area with neurenteric 

 canal as in Graf Spee's embryo, by a ventro-caudal infolding, 

 becomes cloacal membrane, or boundary between the entoderm 

 of the entodermal cloaca and the ectoderm, and having in its. 

 structure at one time both these layers but no mesoderm. 



This leads us now to consider — 



3. The nature of extroversions of the bladder and allied 

 conditions in the same region. 



Many explanations have been advanced as to bladder extro- 

 versions. I need only briefly discuss two before proceeding 

 to indicate the explanation afforded by what has already been, 

 stated. Champneys' paper should be consulted for a full 

 discussion of all the views. 



Professor Wood, of King's College, London, held that the 

 bladder was allantoic in origin. The allantois had its vascular 

 layer (mesoderm) and mucous layer (endoderm). A varying 

 amount of an "inflammatory change or adhesions or some 

 degenerative process .... such as syphilis," he believed, ac- 

 counted for the varieties of this lesion. Wood's explanation waa 



