GENERAL PHEXOMEXA OF DEYELOPMEXT. 



689 



SHORT OUTLINE OF THE MORE GEN^ERAL PHEN"OMEK"A 

 OF THE DEVELOPMENT OF THE OVUM. 



Distinction of Embryonic and Peripheral Phenomena. — From 

 what has gone before it will be seen that the fundamental phenomena 

 of development in the ovum consist essentially in changes which take 

 place in the several layers of the blastoderm. Considered individually 

 and minutel}', they are mainly of the nature of cell multiplication and 

 cell differentiation. Eegarded as a whole tliey may be placed under 

 two divisions, according as 1st, they have their seat in the parts fi'om 

 which the future embryo is formed, and are therefore intra-emlnjonic, or, 

 2nd, are extra-emhrnonic, and connected with the production of other 

 parts, having usually a membranous form, which surround the embryo 

 within the ovum, and form principally the amnion, yolk-sac, allantois, 

 and chorion. It is to be remarked, however, that although in the 

 progress of development all these membranes are mainly peripheral 

 or extra-embryonic in their situation, they are not entirely so in their 

 origin, for one of them — the allantois — springs originally from a part 

 within the body of the embryo ; and all of them, in mammals at least, 

 by the original continuity of the blastoderm, are necessarily united at 

 certain places with parts of the embryo. Hence they have been called 

 foetal appendages or foetal membranes. 



Fiff. 499. 



Fig. 499. — Ovum of the Rabbit from the Uterus (from Kolliker after Bischoff). 

 The ovum was about one seventh of an inch in diameter ; a, the remains of the zona 

 peUucida or external membrane ; b, the vesicular blastoderm ; c, the germinal area ; 

 d, the outer limb of the double layer. 



It is also to be held in remembrance that in birds, the blastoderm, 

 which is originally restricted to the comparatively narrow limits of the 

 cicatricula, extends itself rapidly in the earlier periods of incubation over 

 the surface of the yolk ; while in mammals, the whole yolk is from the 

 first covered by the vesicular blastoderm directly resulting from segmen- 

 tation. In both, however, there may be distinguished a central and 

 peripheral region of the blastoderm, and to the central part, as being 

 the more immediate seat of the development of the embryo and its 

 organs, without attempting to define very closely its limits, the name of 



VOL. II. T Y 



