434 INVERTEBRATA CHAP. 



from the posterior ones, which are the rudiments of the testes. This 

 septum is probably formed as an outwardly projecting fold of the 

 splanchnic mesoderm, possibly it may be a modification of portions of 

 the thin envelopes of the genital cells, which envelopes are themselves 

 thin layers of cells derived from the splanchnic layer of the mesoderm. 



According to Doncaster, the hood which surrounds the head is 

 formed by a splitting off of an outer layer of cells from the ectodermic 

 thickening on each side, and the head coelom persists for a considerable 

 time and it is its outer wall which is metamorphosed into the 

 masticatory muscles. 



The alimentary canal reacquires its cavity about the eighth day, 

 and on this day its posterior end bends ventrally and comes into 

 contact with the ectoderm, where the anus is formed. On the head 

 at the same time there appears a double curved line of closely packed 

 nuclei, in the form of a horse-shoe open backwards, and this constitutes 

 the olfactory organ. 



Doncaster was unable to keep the young Sagitta alive for a 

 longer period than fifteen days, and up till this period no further 

 changes had taken place in the genital rudiments. The further 

 development of these organs was determined by fishing young 

 individuals from the Plankton. 



By a comparison of these with the latest stages reared from the 

 egg it is seen that each of the four primary genital cells divides so as 

 to form a little mass of cells. The masses of cells representing the 

 ovaries grow forwards, those representing the testes grow backwards. 

 Incomplete longitudinal septa appear in the tail coelom on each side, 

 and bunches of cells drop off from the testes and float in the tail 

 coelomic cavity, where they undergo all the changes which lead to 

 their transformation into mature spermatozoa. 



The vasa deferentia appear as ectodermic thickenings in the space 

 between' the lateral fin and the tail fin on each side. This thickening 

 splits into two layers with a cavity between them. In front this 

 cavity is exceedingly narrow, and here it eventually acquires an 

 opening into the coelom. Behind it acquires an opening to the 

 exterior, and this opening is formed at a spot in the course of a 

 longitudinal groove which appears on its outer wall. 



The ovary becomes differentiated into two well-defined layers, 

 an inner and outer, between which is a loose ill-defined mass of cells. 

 The outer layer resembles an epithelium and consists of small cubical 

 cells abutting against the outer wall of the trunk coelom. The inner 

 layer consists of tall columnar cells which are the mother cells of 

 the ova. 



As the ova enlarge and mature they become pressed out of this layer 

 of epithelium, and are found projecting from the surface of the ovary 

 to wards the body-cavity, though covered with a structureless membrane. 

 The loose cells which intervene, especially at the base of the ovary 

 between the germinal epithelium and the ectoderm, eventually develop 

 a cavity and form the oviduct. This grows backwards and at 



