GENERAL EMBRYOLOGY. 



Ccelomic Pouches. When the gastrula of Sagitta has 

 been formed, there arise on the walls of the archenteron 

 opposite the mouth two entoblastic folds, symmetrical with 

 the mid-line of the body, which divide the archenteron 

 into three cavities, at first still in communication the 

 permanent digestive tract and the two rudiments of the 

 body-cavity, the coelomic diverticula. Then the blastopore 

 closes and the entoderm folds extend even into the anterior 

 end of the gastrula, fusing here with the walls. In this 

 way the two coelomic diverticula become completely sep- 

 arated from the digestive tract ; farther, in a corresponding 

 manner, the hitherto continuous entoblast is divided into 

 three epithelial pouches; the middle one is the lining of 

 the digestive tract or the secondary entoblast (glandular 

 lining of the intestine), the lateral ones are the lining of the 

 coelomic sacs, or the paired rudiments of the middle germ- 

 layers. In each of these rudiments two layers are to be 

 recognized, which become separated by the body-cavity ; 

 one layer lies against the digestive tract and is hence called 

 the splanelinopleitre, the other layer comes closely under 

 the ectoblast, forming the external wall of the body, and 

 is called the soinatoplcnrc. From what has been said above 

 it is evident that the epithelial mesoblast 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 Mesepithelium. 

 Now as regards the mode of distribution of the mesenchvme 



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and of the mesepithelium, there are three cases possible, 

 and actually occurring. There are purely mesenchymatous 

 animals, like the flat-worms, and purely mesepithelial, like 

 Sagitta, many annelids, and AinpJiioxns ; but there are 

 also animals in which the mesoblast consists of mesenchyme 

 and mesepithelium : either the mesenchyme arises first and 

 later the mesepithelium, as in the eeJiinoderws, or the 

 reverse order is followed, as in most vertebrates. 



Histological and Organological Differentiation.- -All 

 the organs of an animal arise from the three germ-layers in 



