2 AUGUSTUS G. POHLMAN 



and the doubtful points answered in so far as it is possible. The 

 short literature review covers the essential facts and effort has 

 been made to reduce the description of the material to a concise 

 tabulation. The writer expresses his indebtedness to Prof. 

 Keibel at whose suggestion the development of the later stages 

 in the embr^^ologj' was undertaken, and to Prof. F. P. Mall for 

 the use of his collection of embrj os and for the many courtesies 

 shown him in the Anatomical Laborator}^ of Johns Hopkins Uni- 

 versity. 



Born's article ('93) reviews the earlier development of the cloa- 

 cal region in a very complete manner, and the substance is as 

 follows: The entoderm of the enteron comes into direct relation 

 with the surface ectoderm in the pharyngeal and cloacal membranes 

 during the formation of the head and tail folds. Both of these 

 membranes lose their primitive position and become folded into 

 the substance of the embryo through increase in the surrounding 

 mesoderm. The allantois, which is developed dorsally in the 

 mammalian embryo (human and guinea pig excepted), shifts to a 

 ventral position on the gut, and is gradually displaced from its 

 intimate relation to the yolk sac through increase in the amount 

 of mesodermal tissue. The primitive streak is carried to the ven- 

 tral surface of the body during the formation of the tail fold, and 

 forms the whole or part of the cloacal membrane. Kolliker 

 f'83), Strahl ('83, '84), and Bonnet ('88) believe that the caudal 

 end of the primitive streak is made up of applied layers of ecto- 

 and entoderm, and that it enters as such into the formation of the 

 cloacal membrane. Keibel ('88) argues that this primitive rela- 

 tion of the ecto- and entoderm is lost through interposition of 

 mesoderm; the latter disappearing later with restoration of the 

 original two layered condition. 



The model of the 4.2 mm. human embryo presented by Keibel 

 ('88) shows the hind gut and widely lumened allantois opening 

 cephalward into the caudal entodermal sac or cloaca. Ventrally, 

 this cloaca is limited as far as the dermal navel by the epithelial 

 cloacal membrane. Caudalward, the limit of the cloacal mem- 

 brane comes about by a mesodermic separation of the epithelial 

 layers. The gut segment distal to the lower limit of the cloacal 



