If) AUGUSTUS G. POHLMAN 



.Model A and attains its largest proportions in Model B or at about 

 the time that the Wolffian ducts reach the cloaca. Its relation 

 to the neural tube and to the chorda is most marked at this stage 

 in the development but the histogenesis of this region (shown in 

 ]\Iodel B as a terminal knob) together with its possible connection 

 with the ano-coccygeal body is not definitely known. The de- 

 generative changes have already set in in Model C and are more 

 apparent in Model D. The shrinkage in size does not affect the 

 tail gut evenly throughout its length, but is less marked at the 

 caudal extremity where a cord of cells may be found even after 

 all traces of the primitive connection with the cloaca have disap- 

 peared. In general, the tail gut arises with the tail and accom- 

 panies the tail in its development and resorbtion. It appears at 

 a 3.5 mm. stage and has almost completely disappeared in an 8.0 

 mm. embrj'o. The comparative embryology of this rudimentary 

 and transient segment of the gut deserves careful study. 



THE CLOACA 



Morphologically, we may hardly consider that portion of the 

 gut caudal to the orifices of the allantois and hind gut as cloaca 

 until the arrival of the Wolffian ducts contribute a connection with. 

 the urogenital system, and practically we may not consider the 

 sac as such until the formation of the cloacal membrane indicates 

 how much is cloaca and how much is tail gut. The condition in 

 the development which antedates these structures might be termed 

 the precloacal stage. 



Born states that the allantois arises ventrally in the human em- 

 bryo and in close relation to the yolk sac, and that the allantois 

 is displaced caudalward through growth in the intervening meso- 

 derm. We may agree with the ffi'st part of this statement from the 

 relations demonstrated in the Spee embryo and in Mall's recon- 

 struction of no. 1. The last point, the downward displacement 

 of the allantois, may be seriously questioned. In the formation 

 of the hind gut, either the allantois is displaced caudalward, or the 

 yolk sac is pushed upward on the gut. A comparison of embryos 

 1 and 3 favors the latter view. The development of the hind gut 



