.534 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



Mammalian ovum, and in an historical point of view I must 

 maintain this, though Bischoff * himself ascribes the first know- 

 ledge of the process to Karl Ernst v. Baer. BischofF's description 

 is really the first that stands on a level with our present know- 

 ledge. In that description it is clearly stated that the germ 

 (yolk) within its investing membrane, and independently of it, 

 breaks up into smaller morphological elements. Thus the ovum 

 of the Kabbit, he says in his excellent treatise, during the 

 cleavage of the germ (} f olk) into progressively smaller spheroids, 

 and surrounded by a thick layer of albumen, passes from the 

 oviduct into the uterus. The duration of its passage through 

 the oviduct, from the concordant testimony of De Graaf, 

 Cruikshank, Coste, Wharton Jones, Barry, and Bischoff, appears 

 to be tolerably constantly two days and a half. 



The confidence that Bischoff 's illustrations inspire, leads us to 

 believe that the cleavage of the Mammalian ovum is not equally 

 uniform throughout its whole extent. Within the uterus a 

 cavity is developed also in the Mammalian ovum, which gra- 

 dually increases to such an extent that the cleavage elements 

 compressed at the periphery form a very thin layer enclosing 

 the cavity, or, as it may also be described, constitute the wall of 

 a vesicle. In this condition the little ovum bad already been 

 recognized by De Graaf. He described it as a minute vesicle 

 composed of two membranes. The external membrane, as 

 Bischoff has clearly shown, is the germ sheath (Keimhiille), but 

 the internal is the proper germ membrane (Blastoderm, 

 Keimhaut). Bischoff further described a dark mass, consisting 

 of spheroids, attached to the germ vesicle at some point of 

 its interior. These, he says, are spheroids which are obviously 

 identical with the spheroids proceeding from the antecedent 

 cleavage of the germ. They must thus in the process of cleav- 

 age remain behind those that form the extremely thin and 

 already very clear and transparent wall of the vesicle. Whether 

 the place where these germ cells accumulate is identical with 

 that at which the germinal vesicle subsequently becomes 

 thickened, we must for the present allow to remain undecided. 



At this thickened spot ("germinal elevation," Keimhiigel of 



* Entwickelungsgeschichte des Kanincheneies, p. 66, 1842. 



