366 L. DONCASTEK. 



striation as in tLe muscles of the adult. During the third 

 and fourth days very considerable changes take place, giving 

 rise to a type of structure closely resembling that of the 

 adult animal. Up to this time the body has been solid, 

 containing no cavities whatever, and the first indication of 

 the change is the formation of a cavity in the mesoderm of 

 each side of the body and also in that of the head. At the 

 same time the endodermic septum dividing the two meso- 

 dermic cords from one another becomes thickened in its 

 anterior half, namely, as far back as the genital cells, so 

 marking out the rudiment of the alimentary canal (a/.), 

 though a lumen does not appear in it for several days. In 

 the head, however, a cavity appears in the front end of the 

 alimentary canal, which Avas formed in the embryo by 

 epiblastic invagination, so that a buccal cavity is now present 

 opening by the mouth to the exterior. The rudiment of the 

 hood {" Kappe " or " prepuce ") of the adult has been form- 

 ing as an ectodermal fold, and at this stage has reached 

 considerable proportions, but since it can only suitably be 

 studied in sections it need not be dealt with here. Under it, 

 however, at this stage four or five small hooks make their 

 appearance, the most posterior of which is the longest, while 

 the front one is hardly visible; as they increase in size new 

 ones beu'in to ^ii-ow in front, until the normal nuuiber for the 

 species is reached (PI. 20, hgs. 18, 19, lilib-.). 



The ectodermic swelling in the front of the head, which 

 was already visible in the embryo, has grown larger aud 

 forms the rudiment of the brain [gcuig. ch.), while at the sides 

 of the mouth a rounded body appears on each side, which is 

 seen in section to be the lateral ganglion of llertwig (the 

 vestibular of Grassi). The eyes («.) also appear at this stage 

 as a ])air of minute black specks on the surface of the head, 

 so small that their structure cannot be made out or compared 

 with that of the adult, except that, as in the adult, the black 

 pigment is surrounded by an oval transparent area. The 

 muscles of the head also take their definitive form during the 

 third and fourth days ; before that time the cells have had 



