XIII 



PHYLUM CHORDATA 



41 



alr-a-p br 



stom. 



reel 



orc l er . (i) sexual form ; (2) tailed larva developed sexually 

 from (1) ; (3) first asexual form or " nurse," the direct outcome of 

 (2) ; (4) second asexual form (phorozooid) developed on the 

 cadophore of (3) from buds originating on the ventral stolon ; (5) 

 the young of the sexual form (1) which are developed on the 

 stalk of (4). 



Salpa, like Doliolum, presents a remarkable alternation of 

 generations. In the sexual form, which has already been described, 

 only one ovum is developed. The testis becomes mature later 

 than the ovum, and the latter is impregnated by sperms from 

 the testis of an individual of an older chain. The development is 

 direct and takes place within the body of the parent, the embryo 

 as it grows project- 

 ing into the branchial 

 cavity. The nourish- 

 ment of the develop- 

 ing embryo (Fig. 740) 

 is effected by the 

 formation of a struc- 

 ture the placenta 

 through which a close 

 union is brought p*ric 



about between the 

 vascular system of . 



the parent and that 

 of the embryo. The 

 placenta of Salpa is 

 partly formed from 

 follicle-cells and ecto- 

 derm cells of the 

 embryo, partly from 

 the cells of the wall 

 of the oviduct. Segmentation is complete. The study of the 

 earlier stages is complicated by the very remarkable and unusual 

 circumstance that during segmentation there is a migration in- 

 wards of some of the cells of the follicle and of the wall of the 

 oviduct, which enter the segmenting ovum and pass among the 

 blastomeres. There is uncertainty as to what part these inwardly 

 migrating cells play in the development of the embryo ; but 

 probably they act merely as carriers of nourishment, and become 

 broken up and eventually completely absorbed. 



There is no tailed larval stage, and the embryo develops the 

 muscle-bands and all the characteristic parts of the adult while 

 still enclosed within the body of the parent and nourished by 

 means of the placenta. This sexually-developed embryo, however, 

 does not give rise to a form exactly like the parent, but to one 

 which differs from the latter in certain less important features and 

 notably in the absence of reproductive organs. The sexually 



FIG. 740. Late stage in the development of Salpa, showing 

 the placental connection with the parent, atr. ap. atrial 

 aperture ; 6?-. branchia ; cil. <jr. ciliated groove ; ebl. elseo- 

 blast (mass of tissue probably representing a vestige of the 

 tail) ; end. endostyle ; n. gn. nerve-ganglion ; ees. oesopha- 

 gus ; or. ap. oral aperture ; peric. pericardium ; pi. placenta ; 

 rcct. rectum ; stol. stolon ; stom. stomach. (From Korschelt 

 and Holder, after Salcnsky.) 



