92 BULLETIN OF THE 
ing larva Vigelius (88, Taf. XIX. Fig. 6) represents this tissne 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? This question has not 
been answered by Vigelius. The existence of homopolar stages like that 
represented in his (86) Figure 25, Taf. XXVI., makes it very difficult 
to establish this 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 manière à former une espèce 
de gastrula par épibolie et l’on ne tarde pas à rencontrer des stades d'un 
volume extrêmement exigu et déjà composés d’une couche exodermique 
et d’une masse endodermique libre dans son intérieur, La masse endo- 
dermique s’atrophie rapidement et lon arrive à une petite blastula qui 
succède non pas à un stade composé de cellules radiaires dans lequel 
se forme une cavité centrale, mais qui est issu, au contraire, d’une vraie 
gastrula née par épibolie dans les premiers stades de la segmentation 
et dans laquelle la masse endodermique est déjà 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 blastoccel is quite different from that of those in Bugula, where a 
great Füllgewebe is formed. Ostroumoff (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 
