106 EMBRYOLOGY. 



CHAPTER VI. 



DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 

 (CCELOM-THEORY.) * 



AFTEK the completion of the gastrula stage the processes of develop- 

 ment become more and more complicated, so that the attention of the 

 observer from this time on must be directed to a series of changes 

 which take place at the same time and in various parts of the 

 embryo. For a transformation now ensues, due to the simultaneous 

 folding of both the inner and outer germ-layers, whereby four new 

 chief organs of the vertebrate body are called into existence. Out 

 of the inner primary germ-layer arise (1) the two middle germ-layers, 

 which enclose between them the body -cavity ; (2) the secondary en- 

 toderm or entoblast (Darmdriisenblatt), which lines the secondary 

 intestine of vertebrated animals ; and (3) the fundament of the axial 

 skeleton, the chorda dorsalis, or notochord. At the same time there 

 is developed from the outer germ-layer, as its only system of organs, 

 the fundament of the central nervous system. Since these four pro- 

 cesses in the development are in part most intimately involved in 

 one another, they cannot be separated in their treatment. 



Here again we have to do with a problem which is one of the 

 most difficult in the embryology of vertebrated animals the 

 history of the development of the two middle germ-layers. Not- 

 withstanding a voluminous literature which has grown out of this 

 theme, there are many conditions, especially among the higher 

 classes of Vertebrata, which are not yet explained in an entirely 

 satisfactory manner. We shall therefore enter somewhat more 

 minutely into this topic, which, like the question as to the origin of 

 the two primary germ-layers, possesses a fundamental significance 

 for the comprehension of the organisation of Vertebrates. 



The presentation of what follows will be essentially facilitated, if 

 we allow ourselves a short digression into the history of the develop- 

 ment of the Invertebrata, and take under consideration a case in which 

 the middle germ-layers and the body-cavity are established in a 

 manner similar to that which obtains in the case of Vertebrata, 

 but which is easier to investigate and to understand. Such an 



* In figs. 66-89 the individual germ-layers are represented in different depths 

 of shade, so as to make their relations to one another more evident. The 

 middle germ-layer is darkest. 



