APP.] EIGHT days' EMBRYO. 467 



owing to the fact that the wall of the embryonic 

 vesicle is exceedingly thin, and attached to the ute- 

 rine wall, they are very difficult to obtain whole. 



Cut the uterus transversely on each side of the 

 swellings and piu the pieces so obtained slightly 

 stretched out in small dissecting dishes. Cover the 

 tissue with picric acid solution and allow it to remain 

 untouched for an hour. Then with two pairs of tine 

 pointed forceps carefully tear the uterus longitu- 

 dinally, slightly to one side of the median line of the 

 free side. This operation will necessarily take some 

 time, for but a small portion should be done at once, 

 the picric acid being allowed time to penetrate into 

 that part of the uterus which has been most recently 

 torn open. 



\V ith care, however, the student will be able to 

 open completely the swelling and will observe within 

 the thin walled vesicle. Great care must also be 

 exercised in freeing the vesicle from the uterus. 



This dissection should be performed with the aid 

 of a dissecting lens. In case the embryonic vesicle 

 is burst it will still be possible to extract the embryonic 

 area which lies on the mesometric side of the uterus ; 

 the area itself is not attached to the uterine walls. 



B. Examination of surf ace view. 

 Observe : 



1. The increased size of the embryonic area. 



2. In the anterior region the medullary folds; di- 

 verging behind and enclosing between them, 



3. The primitive streak. 



4. The area opaca now completely surrounding the 

 embryo. 



302 



