172 



EMBRYOLOGY 



the intestinal wall. Immediately before the secondary for- 

 mation of the intestine takes place, the embryo would be to 

 a certain extent in the condition of the acoelous Turbellaria, 

 in which the food-bodies pass directly into the body paren- 

 chyma. To be sure, an intestinal cavity would exist in the 

 embryos, but it would be limited by the body parenchyma. 

 Should these observations be confirmed, they might perhaps 

 throw some light on the establishment of the conditions 

 which exist in the Acosla. 



B 



Fig. 85. — Sections through embryos of Drndroca-liimlactcum (somewhat diagram- 

 matic, after Hallez). Ec, ectoderm; En, entoderm ; Dj, yolk-cells ; PK, provisional 

 embryonal pharynx and (in Fig. C, Ph") permanent pharynx ; Wz, migratory 

 cells. 



The branched form of the intestine of Triclads arises in a 

 similar way to that of the Polyclads, i.e., by the ingrowth of 

 connective-tissue septa from the periphery toward the middle 

 line. This tissue, like the body musculature, owes its origin 

 to the migratory cells, from which the sexual organs likewise 

 arise (Iijima). 



The fundament of the nervous system was found by the three authors 

 mentioned lying deep in the body tissues, and they could not discover 

 that it had any connection with the ectoderm. If the statement of 



