136 EDWARD MCCRADY, JR. 



in the pancreas. Pleuroperitoneal membranes. The ureteric 

 bud. The primitive choanae. The motor nerves of the eye. 

 Miscellaneous details. 



External distinctive features. Stage 31 is distinguished 

 from stage 30 by the loss of the naso-oral groove, the develop- 

 ment of the frontal process, the auricular tubercles, the fusion 

 of the mandibular and hyoid arches beneath the external 

 auditory meatus, the appearance of prominent digital ridges 

 on the forefoot, the enlargement of the allantois until its 

 diameter is about one-third the greatest length of the body. 

 It is distiguished from stage 32 by the smaller allantois, the 

 fact that the digital anlagen on the forefoot are not yet buds ; 

 and the hind limb is still a bud, not a club, that is, its length 

 is not appreciably greater than its width and it has no 

 terminal enlargement. 



Changes in the chorion. Up until the middle of the tenth 

 day all of the vesicles are spherical and roll or float about 

 freely in the cavity of the uterus. During stages 29 and 30 

 the animal pole of the vesicle comes to rest against the uterine 

 mucosa. This relation is consistent in all of the vesicles, as if 

 the area vasculosa were for some reason sticky and inclined to 

 adhere to the mucosa. Selenka (1887, S. 128) was of the 

 opinion that the shell membrane (which he erroneously called 

 the 'granulosa membrane') over the area vasculosa becomes 

 soft and adhesive at this time, and acts as the anchoring agent. 

 At any rate, the vesicles all come to rest in this position with 

 the area vasculosa against the mucosa and the non-vascular 

 chorion on the free side. Osborn (1883 a and b, 1887) de- 

 scribed the vesicles at this time as aligned in two longitudinal 

 furrows of the uterine mucosa, but no such arrangement has 

 been seen by any other investigator. The vesicles are usually 

 scattered at random throughout the uterus. 



After coming to rest the vesicle continues to enlarge, but 

 the enlargement is accommodated in different ways at the 

 attached and unattached faces of the vesicle. The attached 

 face wrinkles into great folds which sink into corresponding 



