1 66 BIOLOGICAL LECTURES. 



the given Anlage-material a rather long skeletal structure with 

 a thickening at its proximal end ; but in other respects this 

 formation is due to dependent differentiation, since the finer 

 details of structure, like the surfaces of the joints and the 

 three-sided shape of the diaphysis, are conditioned by the 

 operations of neighboring parts. 



The segmentation of the common arborescent glands into 

 lobules appears to be conditioned by the formative operations 

 of the epithelia and hence of the specific parts, and, so far as 

 this is true, the segmentation is a self-dijfe7'entiatio7i of the 

 glandular substance. In the liver, however, which is a reticu- 

 lar gland, the normal size and form of the lobules and also the 

 lobular segmentation itself appears to be conditioned by the 

 blood-vessels — on the one hand by the requisite length of the 

 capillaries, and on the other by the peculiarity in the ramifica- 

 tion of the portal vein, which during its growth develops 

 dichotomic branches in its capillary network. Hence the acinous 

 segmentation of the liver parenchyma represents a differentiation 

 of the glandidar substance depending on the vascular system. 



After, or at the same time as the actual ascertainment of 

 such ^^ local'' conditions of tJie fonnative causes, we shall en- 

 deavor to look for factors which condition the magnitude and 

 direction of the formative processes ; simultaneously, or even 

 before this, we may be able to ascertain also the time when 

 many of these formations are reduced to a norm, as, e.g., the 

 time when the direction of the median sagittal plane of the em- 

 bryo is determined ; for it is not necessary that these forma- 

 tive conditions be first conditioned when the ultimate forms 

 first become visible. 



On the contrary, in the perfectly normal, i.e., perfectly typical, 

 course of the individual development, all the typical structures 

 must at the very latest be in some way conditioned in the 

 fertilized q^^, either implicite in their earliest components, or 

 explicite in already visible Anlagen. Nevertheless, we must 

 assume that there is really no such thing as perfectly typical 

 development (12), but that in every individual development 

 greater or less disturbances take place, which are compensated 

 by the putting into action of regulating mechanisms. Accu- 



