172 



EMBRYOLOGY 



the intestinal Avall. Immediately before the secondary for- 

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

 a certain extent in the condition of the accclous Tarbellaria, 

 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 tlie body parenchyma. 

 Should these observations be confii^med, they might perhaps 

 throw some light on the establishment of the conditions 

 which exist in the Acrela. 



A B 



Fig. 85.— Sections through embryos of Dnidi-oca'liim lacienm (somewhat diaffram- 

 matie, after Hai.lkz). Ec, ectoderm ; En, entoderm ; Dz, yolk-cells ; Ph', provisional 

 embryonal pharynx and (in Fig. C, Ph") permanent jiharynx ; IF:, migratory 

 cells. 



Tlie 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 

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



