ZOOLOGY. 469 



early period ; the blastopore appears to close from before backwards. 

 The lower layer, which goes to form the eudoderm, does not correspond 

 to more tban one-third of the blastula. This undergoes invagination ; 

 the lluid of the cleavage ca\'ity becomes absorbed, and bilateral sym- 

 metry soon becomes well marked." 



In the "'third period' the primitive segments, the nervous system, 

 and the notochord begin to be apparent ; the remnant of the blasto- 

 pore persists as an opening between the enteric cavity and the nerve- 

 tube, representing the typical neuro-enteric canal. Contemporaneously 

 with the development of the nerve-tube, the mesoderm develops the 

 primitive segments ; two lateral longitudinal folds arise in the dorsal 

 portion of the endoderm,and represent the rudiments of the mesoderm. 

 The cavities of the primitive segments are diverticula from the arch- 

 enteric cavity." 



In the fourth histological differentiation especially supervenes. "The 

 muscles become apparent ; the notochord undergoes histological differ- 

 entiation, and fibrous cords appear in the medullary tube. At the same 

 time the larva alters greatly in form, becomes elongated and compressed, 

 and takes on generally a piscine character. The increase in the num- 

 ber of primitive segments goes on but slowly, but what are formed 

 gradually fuse in the median central line. Each muscle-cell has at first 

 only a single fibril, which is continuous throughout the length of the 

 body." 



In the fifth period " those changes occur which enable the embryo to 

 pass into the larva. A number of orifices are now formed — the mouth 

 and the"first gill-cleft, the orifice of the ciliated organ (or left endodermal 

 sac), the club-shaped gland, and the anus. The body meanwhile in- 

 creases in length, fresh segments being formed ; a number of strong 

 motile flagella may be seen to be developed from the cells, and all the 

 tissues of the body are now formed of transparent protoplasm." 



The first four of these phases successively manifested correspond to 

 the first of " two well-marked stages, the one embryonic when it is ef- 

 fected at the cost of the nutrient material contained in the egg, and is 

 very rapid. The fifth phase represents the second stage. At the close 

 of the first the mouth is developed, and the first gill-cleft. Tlie larva 

 now begins to feed itself; its cells contain transparent protoplasm, and 

 the developmental processes are very much slower." (J. R. M. S., (2) II, 

 174-176, from Arbeit. Zool. Inst. Univ. Wien, IV, 1-89, with pi.) 



