EMBRYONIC FISSION IN OYOLOSTOMATOUS POLYZOA. 3 



present in an ovicell at any one time may exceed one hundred, 

 and these have all been produced by budding from the above- 

 described " primary embryo." 



(xiii) Each of the "secondary embryos" acquires its well- 

 known two-layered condition at the time of its separation from 

 the budding mass of embryonic cells. It develops in a vacuole 

 of the protoplasmic reticulum, which presumably supplies it 

 with nutriment since the embryo rapidly increases in size, be- 

 coming ciliated externally, and ultimately escaping through 

 the tubular aperture of the ovicell as a characteristic Cyclo- 

 stome larva. 



Taking the above history into consideration, it is not sur- 

 prising that, as is actually the case, the Cyclostome larva 

 differs considerably in structure from that of other marine 

 Polyzoa. This history also explains the fact that no observer 

 has ever succeeded in giving an account of any process corre- 

 sponding to egg-cleavage in Cyclostomata. 



The protoplasmic mass surrounding the embryos has been 

 figured by Smitt (34),^ who has alluded to the yellow colour 

 so characteristic of the contents of the ovicell. This colour is 

 contained principally in the protoplasmic reticulum, although 

 the embryos themselves have a yellowish colour. 



The first satisfactory account of the Cyclostome larva was, 

 however, given by Barrois (1), who calls special attention to 

 the fact that no previous observer had been able to discover 

 "genital products" in any Cyclostome, and adds, " Je n'ai 

 pour ma part encore reussi qa'k suivre les morulas jusqu'a 

 des stades composes d'un nombre d'elements de moins en 

 moins nombreux et plus volumineux, sans r^ussir encore a 

 constater d'une maniere bien certaine la presence de I'oeuf ; " 

 although supposing that the Cyclostomes do not really differ 

 from other Polyzoa in this respect.^ Barrois^ failure to under- 

 stand the early development of the embryos is readily explained 

 if my own account be correct ; and it is not surprising, con- 



' See his pi. iv, fig. 2. 

 ^ L. c, pp. 58, 59, note. 



