GENERAL EMBRYOLOGY. 



the entodermal folds extend to the opposite side, where they fuse 

 with the walls (B). In this way a pair of coelomic pouches are 

 cut off from the rest of the archenteron which forms the lumen of 

 the digestive tract and its derivatives, while the walls of the 

 pouches form the mesothelium, that of the digestive region the 

 secondary entoderm. In each coelomic pouch two walls are recog- 

 nizable, an inner or splanchnic layer which unites with the 

 entoderm to form the wall of the digestive tract, the splanchno- 

 pleure, while the somatic layer unites similarly with ectoderm to 

 form an outer body wall, the somatopleure. From the foregoing it 

 is evident that the mesothelium is strictly not a single layer, but 

 consists of two layers which pass into each other, and that its 

 origin is closely connected with the formation of the body cavity. 



Occurrence of Mesenchyme and Mesothelium There are three 

 possible methods for the distribution of mesenchyme and meso- 

 thelium, and these actually occur. There are purely mesenchy- 

 matous animals, like the flat-worms, and purely mesothelial, like 

 Sagitta, many annelids, and Amphioxus; but there are also- 

 animals in which the mesoderm consists of mesenchyme and 

 mesothelium: either the mesenchyme arises first and later the 

 mesothelium, as in the echinoderms, or the reverse order is fol- 

 lowed, as in most vertebrates. 



Histological and Organological Differentiation. All the organs 

 of an animal arise from the three germ-layers in this way : first, 

 embryonic cell material is marked off into separate complexes, 

 usually by infolding (organological differentiation), and then later 

 these become changed into tissues (Mstological differentiation). 

 The details differ in the various animal groups; the following is 

 the most general : from the ectoderm arise the skin with its glands 

 and appendages, the nervous system, and the sensory epithelium^ 

 the entoderm gives rise to the most important part of the digestive 

 tract with its glands; while muscles, blood, supporting and con- 

 nective substances, excretory organs, in whole or in part, arise in 

 the mesoderm; the sexual organs are also usually mesodermal. 



Relations of the Germ-layers in Budding. Of late the question has often 

 been raised as to how far the germ-layer theory is applicable to the occur- 

 rences in asexual reproduction. At first one would expect in budding, 

 and still more in the case of division, that each organ of the daughter 

 animal would arise from the corresponding organ of the maternal animal, 

 or, if that be impossible by conditions of space, from a mass of tissue 

 belonging to one of the same germ-layers. In many instances this is cer- 

 tainly the case, as, for example, in the budding of hydroids the entoderm 

 and ectoderm of the bud arise from the corresponding layers of the maternal 



