114 George Lefevre 



4 Formation of Later Embryo and Larva 



Later cleavage stages, the formation of the blastula and gas- 

 trula, and the differentiation of the early trochophore will be con- 

 sidered in the chapter dealing with the observations on preserved 

 material, as most of the developmental changes, which can be de- 

 termined with accuracy in the parthenogenesis of Thalassema, 

 must be made out from an examination of sections and total 

 preparations. 



As Torrey has stated {op. cit., p. 187), cilia first appear on the 

 normal blastula with great regularity at four and one-half hours 

 after fertilization, and simultaneously on the prototroch and 

 rosette. Although I have occasionally observed the appearance 

 ot the cilia of the prototroch and the apical flagella on the parthe- 

 nogenetic embryos at the same time, t. e., four and one-half hours 

 after removal from the solutions, it is more usually the case that 

 their formation is considerably delayed and they are not seen for 

 from six to nine hours. In the few experiments where the cilia were 

 first observed at the normal time, the rate of development from 

 the first cleavage onward coincided very closely with the normal 

 rate. In favorable experiments, the cilia show perfectly typical 

 relations; those of the prototroch are at first short and delicate and 

 form a broad band completely encircling the embryo, while the 

 apical cilia soon become quite long i;nd project in front as a pencil 

 of rather stiff straight flagella. Torrey's description of the normal 

 cilia at this stage corresponds in all respects with the observations 

 which I have repeatedly made on the parthenogenetic embryos, 

 and may be quoted: "At first the cilia on the prototroch are uni- 

 form in size, but during the differentiation of the trochophore, 

 there appear two bands of longer cilia — one at the upper edge 

 which beat actively, and a narrower one on the lower edge which 

 hang down and move more slowly and indefinitely. Between 

 these two rows the shorter cilia are retained. The long flagella, 

 borne entirely by the rosette, are about 20 in number and when 

 the embryo is actively swimming are carried stretched out in front 

 and bunched closely together, quite as in a pilidium larva. When 

 the animal is at rest the flagella wave about slowly. In the trocho- 



