642 L- ß- Cary, 



When tlie embiyo has reached the age at which the primordia 

 begin to be laid down, the differentiation is very rapid. The ex- 

 cretor}" bladder and the main trunks of the water vascular system 

 make their appearence, closely following- upon the differentiation 

 of the primordium of the gut. The brain and the main branches 

 of the nervous system become recognisable at about the same time. 

 The cells which are to form the ventral sucker become marked off 

 from the surrounding meristem cells: the large vacuolated cells 

 along the ventral part of the body (Blasenzellen of Schwarze) make 

 their appearance; and the cuticular gland cells become sharply 

 defined. The cells from which the reproductive organs are to be 

 developed form a large mass in the central part of the bodj-, in 

 which there can soon be recognised the primordia of the ovary, the 

 testes and the accessory portions of the reproductive system. 



The development of the digestive system. 



The development of the digestive system, whether the primordium 

 of the gut has come over from the primitive endoderm cells as a 

 recognisable cell mass, or is diff'erentiated from a homogenous syn- 

 cytium is the same. The endoderm cells become arranged into a 

 solid rod which is surrounded by the meristem cells. At this stage 

 in the development the primordium has much the same appearance 

 a has been figured by Rossbach for that of the redia. 



Schwarze (1886) and Looss (1892) describe the primordium of 

 the intestine as arising at the anterior end of the embryo, where 

 the mouth opening is to be established, and from the there growing 

 backward to the posterior limits of the caeca. In Diplodiscus the 

 connection of the gut with the mouth opening is established through 

 the growning backward of the pharynx and oesophagus, which come 

 from the ditterentation of the primordium at the anterior end of 

 the body : from the ectoderm. Schwärze, flg. 4, shows what I believe 

 to be the last mentioned structure, which he considered as the 

 Avhole of the digestive system. 



The lumen of the gut becomes established by the rearrange- 

 ment of the cells about a central cavity. For a time the gut con- 

 sists of a straight hollow tube until it becomes connected with the 

 oesophagus by the breaking through of its wall at the anterior end. 

 The division of the gut into the two caeca and the backward 

 growth of the latter takes place after the lumen has been formed. 

 At first the postei-ior end of the primordium becomes broadened and 



