80 CHANGES IN THE MAMMIFEROUS OVUM 



of the yolk must, in this case, therefore, he regarded merely as a nutritive 

 fluid. 



According to Reichert's account of the development of the emhryo frog,* 

 it would seem that the first act of the process consists in the formation at 

 the surface of the ovum of a fine membranous layer of cells, which, by 

 extending over the whole surface, shortly constitutes a complete invest- 

 ment surrounding the yolk. And, in his opinion, it is not until this 

 investing membrane is completely formed, that the development of the 

 first formed parts of the embryo namely, the two oval masses and the 

 primitive groove between them commences. From the observations of 

 Vogt,f however, it appears that, in the development of the toad, no such 

 investing membrane is formed. The earliest peripheral yolk-cells, it is 

 true, unite together so as to form a kind of membrane at the surface of 

 the yolk, but, as before observed, the primitive groove and laminae dorsales 

 are developed in the substance of this layer of cells and not beneath it. 

 And it is only at a later period of the development of the embryo, that 

 anything like an investing membrane is formed over its exterior, and this 

 consists simply of a layer of pavement epithelium. A like absence of 

 investing membrane was also observed by him in the ova of the salmon. J 

 So that either Reichert's account must be considered erroneous, or, it 

 must be concluded, that, in its earliest stages of development, the embryo 

 pursues a different course in frogs, than in other amphibia and than in 

 fishes. 



Further information relating to the development of the several parts of 

 the embryo of amphibia and fishes, is contained under the " Development 

 of Organs." 



The information we are in possession of, in relation to the changes 

 ensuing in the ovum of Mammalia^ at this stage, is derived almost 

 exclusively from observations made by Bischoff upon the ova of rabbits 

 and bitches. Some few of the facts to be here mentioned have been 

 already stated in Miiller's Physiology || but it will be necessary to repeat 

 them in order to preserve the continuity of the account. 



About the time at which the mammiferous ovum reaches the uterus, 

 the process of division and subdivision of the yolk appears to have 

 ceased, its substance having been resolved into its ultimate and smallest 

 divisions, while its surface presents a uniform finely-granular aspect, 



* Miiller's Physiology, p. 1521. 



t Entwickelungs-gesch. der Geburtshelferkrb'te, p. 32. 



Hist. Nat. des Poissons, t. i. p. 48. 



Respecting the development of Birds no new facts of importance have recently been 

 added to our knowledge. A series of plates illustrating the changes which the embryo of 

 the chick undergoes during its development have been published by Prof. M. P. Erdl. 

 (Entwickelung des Menschen und des Hiinchens im Eie. 2. Bd. Leipsic, 1845.) 



|| pp. 1560-64. 



