FORMATION OF GERM LAYERS 115 



early embryo are compensated for or neutralised in the 

 course of further development. Baer in this shows himself a 

 vitalist. 



It is, however, the third and subsequent Scholia which 

 must here particularly occupy our attention, for it is in these 

 that von Baer comes to grips with morphological problems. 

 Already in the second Scholion he had definitely enunciated 

 the law which runs as a theme throughout the volume, the 

 observational and the theoretical part alike, the law that 

 development is essentially a process of differentiation by 

 which the germ becomes ever more and more individualised. 

 " The essential result of development," he writes, " when 

 we consider it as a whole, is the increasing independence 

 {Selbstdndigkeif) of the developing animal" (p. 148). In 

 the third Scholion he elaborates this thought and shows 

 that differentiation takes place in triple wise. The three 

 processes of differentiation are "primary differentiation" or 

 layer-formation, " histological differentiation" within the 

 layers, and the "morphological differentiation" of primitive 

 organs. 



The first of these differentiations in time is the formation 

 of the germ-layers, which takes place by a splitting or 

 separation of the blastoderm into a series of superimposed 

 lamellae. Baer's account of the process in the chick is 

 as follows : 



" First of all, the germ separates out into heterogeneous 

 layers, which with advancing development acquire ever 

 greater individuality, but even on their first appearance show 

 rudiments of the structures which will characterise them 

 later. Thus in the germ of the bird, so soon as it acquires 

 consistency at the beginning of incubation, we can 

 distinguish an upper smooth continuous surface and a lower 

 more granular surface. The blastoderm separates thereupon 

 into two distinct layers, of which the lower develops into the 

 plastic body-parts of the embryo, the upper into the animal 

 parts ; the lower shows clearly a further division into two 

 closely connected subsidiary layers the mucous layer and 

 the vessel-layer ; the original upper layer also shows a 

 division into two, which form respectively the skin and the 

 parts which I have called the true ventral and dorsal 



