352 CHESTER H. HEUSER 



beginning of segmentation of the ovum. Selenka ('87) calcu- 

 lated that segmentation did not begin until the end of the fifth 

 day, but from much more extensive material Hartman ('19) 

 estimates that the first division of the fertilized ovum occurs 

 two days earlier. During the next three days the embryo devel- 

 ops to a stage with an undifferentiated fore- and hindgut. From 

 this primitive condition the digestive tract, during the last five 

 days before birth, is rapidly transformed to a functioning state. 

 This rapid change from the primitive gut into an alimentary 

 system capable of digesting milk, even at a stage of relatively 

 incomplete differentiation, presents many problems of great 

 interest. 



The stomach of the late embryo may be described as consisting 

 of a cardiac and a pyloric portion, more or less arbitrarily marked 

 off on the lesser curvature at the angular incisure. The cardiac 

 portion, using the terminology adopted by F. T. Lewis ('12), 

 may be said to include a prominent blind pouch or fundus taper- 

 ing rapidly to a tip and a large lower part or corpus. The differ- 

 ent parts blend imperceptibly with each other along the greater 

 curvature. In the opossum embryo the pars pylorica is not 

 tubular in form as described by Lewis for the human embryo, 

 and as Lewis and I reported ('14 and '15) for the cat, rat, pig, 

 and sheep. In a younger opossum embryo — one measuring 7 

 mm. in length — the whole digestive tract by reconstruction 

 (Heuser, '18) showed a more elongated portion leading into the 

 duodenum, and the stomach resembled the organ in the 10-mm. 

 human embryo (Lewis, '12, fig. 5). The gastric canal, which 

 Lewis demonstrated to be an epithelial differentiation in human 

 embryos, and as we likewise found to be the case in the other 

 mammals named, has not been definitely made out in the opossum 

 embryos examined; however, it is believed that it would be 

 revealed by wax reconstructions of the gastric epithelium in a 

 proper series of stages. 



The stomach is small in the late embryo and the lumen is 

 further reduced in size by the presence of folds or rugae in the 

 mucosa. The rugae vary considerably in number and promi- 

 nence in embryos of this stage, being less conspicuous in two other 



