10 AUGUSTUS G. POHLMAN 



The description of the models has been made in as brief a man- 

 ner as feasible, and a discussion of the various points in question 

 will be brought up after they have been described. All of these 

 models represent epithelial casts, and all of them with the excep- 

 tion of Model F. are X 100. 



Model A. {No. 3; Mall no. 164; 3.5 mm.) This model shows 

 the cloaca to be a sort of conical sac receiving the wider lumened 

 hind gut dorsally, and the narrow lumened allantois ventrally. 

 The latter joining the cloaca at about a right angle. The cloaca 

 is much wider dorsally and is somewhat wedged-shaped in sec- 

 tion. Ventrally, it is limited for about one-half of its extent by 

 the epithelial cloacal membrane which is indicated by a surface 

 contact. The right wall of the cloaca is concaved from before 

 backwards while the left side (shown) is correspondingly convex, 

 and presents a well marked furrow extending from the saddle 

 between the allantois and hind gut to about the mid area of the 

 cloacal membrane. This furrow is interesting because while it 

 indicates the probable line of the later division, it was found in 

 but two embryos of the series examined. The part of the cloaca 

 caudal to the cloacal membrane — the tail gut — is quite short and 

 cone-shaped. The model with the exception of the furrow con- 

 forms to Keibel's model of the His embryo EB (3 mm.) 



Model B. {No. 8; Mall no. 186; 3.5 mm.) This model, 

 although made from an embryo of the same length as the above, 

 shows a distinctly older stage in the development of the tract. 

 The allantois has a much wider lumen and the angle between it 

 and the hind gut is much more acute. The Wolffian ducts have 

 reached the cloaca, and attach but do not open into it ventrally 

 near the upper limit of the cloacal membrane. The tail gut is 

 much larger both in length and in thickness, and terminates in an 

 undifferentiated cell mass formed by itself, the chorda and the 

 neural tube (shown as a knob-like ending). The cloaca is 

 wider dorsally than ventrally. A stage similar to the one de- 

 scribed was found in two other embryos (nos. 7 and 8), in both of 

 which the allantois was wide at its apparent opening into the cloaca. 

 The possibility of some minor degree of development abnor- 



