MUSEUM OF COMPARATIVE ZOOLOGY. 319 



This organ, for which Krohn has proposed the name " elceoblast," 

 attains its greatest development in the embryo, and begins to disap- 

 pear as soon as the chain begins to form within the body, and is there- 

 fore provisional, as pointed out by Krohn, and has no permanent place 

 or function in the adult. Meyen regarded it as a yolk (p. 401), and 

 there seems to be little doubt that it is, if not strictly yolk, at least 

 something having a very -similar use, and supplying the material which 

 is to be employed in the formation of the zooids of the chain. Leuckart 

 states (p. 57), that it is found in both forms of Salpa, but this is not 

 the case. It is always present in the solitary Salpa during the em- 

 bryonic development, and the nucleus of the young chain-salpa di- 

 vides, like that of the solitary form, into an upper smaller portion, 

 which gives rise to the digestive organs, and a lower, larger portion, 

 which thus agrees with the elaxiblast in its mode of origin, and, during 

 the earliest stages, in appearance also (Fig. 32, t). It never attains 

 a great size, as compared with the other organs, and soon forms 

 a compact globular mass of cells (Fig. 34, t), which in the adult 

 chain-salpa forms the testis (Fig. 5). Since the testis of the chain- 

 salpa agrees so closely with the elaeoblast of the solitary one, it seems 

 very probable that this is also a reproductive organ, and that Mej-en 

 was right in calling it a yolk-deposit. It is not a true ovary., for this 

 originates in a different way, as will be shown further on.* 



The Atrium. — The adult Salpa is composed of three principal tunics, 

 each enclosing a special cavity. The formation of the-outer one of 

 these, the outer tunic, has already been described, and we have seen 

 that it is derived from the outer layer of the gastrula ; its cavity has 



* Few organs have had their functions more disputed than this elaeoblast. Many of 

 the earlier observers considered it a liver, and Sars repeats this error, although Meyen 

 had previously determined its connection, in some way, with development. Krohn 

 discovered that it was transitory and provisional, but does not commit himself in regard 

 to its function. Huxley described the true liver, and, although acquainted with all that 

 had been written upon the elaeoblast, says (p. 571), "There would seem to be no clew 

 either to the homology or to the function of the elceoblast. Without hazarding a con- 

 jecture, it may be remarked, as a curious fact, that these animals, so remarkable for 

 possessing in the foetal state a true though rudimentary placental circulation, possess 

 an organ which in structure and duration somewhat calls to mind the thymus gland." 

 Leuckart (p. 57) concludes that it is a depot of nutriment stored up for future use, but 

 he does not state whether it is to lie used in the formation of the embryo or in the de- 

 velopment of the chain. We will return to the subject of its homology with the testicle 

 further on. 



