86 EMBRYOLOGY. 



germ-layers, and are distinguished according to their positions as the 

 outer (ak) and the inner (ik). Whereas in the blastula the individual 

 cells differ only a little from one another, with the process of gastru- 

 lation a division ot labor begins to assert itself, a fact which may 

 be recognised in the case of the free-swimming larvae of Inver- 

 tebrates. The outer germ-layer (ak) (also called ectoUast or ectoderm) 

 serves as a covering for the body, is at the same time the organ of 

 sensation, and effects locomotion when cilia are developed from the 

 cells, as is the case with Amphioxus. The inner germ-layer (ik) 

 (entoblast or entoderm) lines the ccelenteron and provides for nutri- 

 tion. The cell-layers thus stand in contrast to each other both as 

 regards position and function, since each has assumed a special duty. 

 In view of this fact they have been designated by C. E. VON BAER 

 as the two primitive organs of the animal body. They present us 

 with a very instructive, because very simple, illustration of the 

 manner in which two organs originate from a single fundament. 

 By invagination the undifferentiated cells of the surface of the 

 blastula are brought into different relations to the outer world, and 

 have consequently been compelled to follow different courses in their 

 development, and to adapt themselves to special duties corresponding 

 to the new relations. 



The separation of the embryonic cell-material into the two primi- 

 tive organs of VON BAER is of decisive significance for the whole 

 subsequent course of the development of the individual cells. For a 

 very definite portion of all the ultimate organs of the body is refer- 

 able to each of the two primitive organs. In order to put this im- 

 portant condition in the proper light at once, let it be stated that the 

 outer germ-layer furnishes the epithelial covering of the body, the 

 epidermis with the glands and hair, the fundament of the nervous 

 system, and that part of the sense organs which is functionally most 

 important. On this account the older embryologists imposed upon it 

 the name of dermo-sensory layer. The inner germ-layer, on the 

 contrary, is converted into the remaining organs of the body into 

 the intestine with its glands, into the body-cavity, into the muscles, 

 etc. ; by far the greater mass of the body, therefore, is differentiated 

 out of it, and it has to pass through the most numerous and the most 

 trenchant metamorphoses.* 



* The practice of distinguishing the outer and the inner germ-layers as animal 

 and vegetative, which was formerly in vogue and is followed even now, is not 

 proper, and ought therefore to be given up. For the transversely striped muscu- 

 lature of the body, which belongs to its animal organs, does not arise from 



