92 BULLETIN OF THE 



ing larva Vigelius ('88, Taf. XIX. Fig. G) represents tins tissue as hav- 

 ing almost entirely disappeared ; that which remains giving rise to the 

 mesodermal lining — the outer layer of the bud — of the developing 

 polypide. 



There can be no doubt that the so-called oral pole of the Bugula larva 

 corresponds to the mouth-bearing pole of Alcyonidium, but does it cor- 

 respond to the pole of ingression of entoderm 1 Tliis question has not 

 been answered by Vigelius. The existence of homopolar stages like that 

 represented in his ('8G) Figure 25, Taf. XXVL, makes it very difficult 

 to establish tliis doubtful point. 



The formation of the inner layer of Cyclostomes has been studied by 

 Barrois ('82, p. 141). He says: " Des les premiers stades les spheres 

 vitellines glissent les unes sur les autres de maniere a former une espcce 

 de gastrula par epibolie et I'on ne tarde pas a rencontrer des stades d'un 

 volume extvemement exigu et dijS, composes d'une couche exodermique 

 et d'une masse endodermique libre dans son interieur. La masse endo- 

 dermique s'atrophie rapidement et I'on arrive a une petite blastula qui 

 succede non pas a un stade compose de cellules radiaires dans lequel 

 se forme une cavite centrale, mais qui est issu, au contraire, d'une vraie 

 gastrula nee par epibolie dans les premiers stades de la segmentation 

 et dans laquelle la masse endodermique est d^ja disparue." I have 

 quoted Barrois thus at length, since his description will show forcibly at 

 least one thing, that the fate of the cells which by ingression had entered 

 the blastocoel is quite different from that of those in Bugula, where a 

 great Fiillgeivebe is formed. Ostroumoflf ('87, p. 183), however, has 

 shown that the inner layer of the Cyclostome larva does not disappear, 

 but comes to line the ectoderm as a very thin layer. In the adult larva, 

 however, we find the contents of the ectodermal sac " filled with me- 

 senchymatous cells, which are commingled with yolk granules and glob- 

 ules of albumen." It is these cells that produce the very considerable 

 mesodermal layer of the first polypide, which arises in the metamorphosis 

 of the larva. Here, as elsewhere, an apparently homopolar stage inter- 

 venes between gastrulation and the formation of larval organs, making 

 orientation difficult. 



Thus, passing from Cyphonautes, through Alcyonidium and Flustrella, 

 Bugula, and finally Cyclostomes, we have a series in all of which the 

 inner germ layer is derived from one pole by ingression or by * epiboly," 

 and in which there is a gradual reduction of the functional entoderm until 

 it seems, in Cyclostomes, to be lost, and a gradual transformation of the 

 mesoderm from a cell mass nearly filling the larva, and producing muscles 



