386 L. DONCASTEH. 



which are aggregated into dorsal and ventral bands on each 

 side, and in these regions some at least of the cells are 

 already elongated and form the rudiments of the mnsclos. 

 The genital cells lie in the mesoderm against its inner wall, 

 .at a point about halfway between head and tail. The endo- 

 derm forms a thin septum stretching from neck to tail, and 

 separating the two halves of the mesoderm. In the head the 

 ectoderm is thickened dorsally and anteriorly to form the 

 brain, and laterally to give rise to the hood, while ventrally 

 it is invaginated and forms the solid rudiment of the mouth 

 and pharynx. The two masses of cephalic mesoderm lie at 

 the sides of the latter, and meet one another dorsally. 



As development proceeds the fins increase in size, and the 

 paired ones at the sides become distinct from that of the tail; 

 the ganglia become more sharply defined, the endoderm of the 

 body increases in thickness as far as the genital cells, and the 

 muscle-cells become definite muscles with transverse striation, 

 and appearing pinnate in section. The epidermal cells of the 

 neck and those just behind the ventral ganglion assume a 

 vacuolated and parenchymatous nature. In the head the 

 hood is formed by splitting, the mouth cavity opens, the 

 vestibular ganglia are formed from ectodermal cells at its 

 sides, and the eyes and hooks appear. The greater part of 

 the cephalic mesoderm cells become muscles, and then a 

 cavity, probably truly coelomic, appears between them and 

 the pharynx. At the same time ccelomic cavities appear in 

 the trunk, and the genital cells migrate across from the 

 splanchnic to the somatic layer, forming as they go the 

 posterior transverse septum, which is probably produced 

 from the mesodermal envelopes of the genital cells. 



The ccelomic cavities increase in size and their boundaries 

 become definite, but it is not absolutely certain whether they 

 are enclosed by a definite epithelium, or by the cells, the 

 outer ends of which form the muscle-fibres ; the latter view 

 seems, however, more probable, since the nuclei bounding the 

 cccloin occur always in direct relation with the pinnate groups 

 of muscle-fibres. The alimentary canal acquires a ciliated 



