96 BULLETIN OF THE 



the most widely separated groups of tlic animal kingdom, the idea that 

 a functional alimentary tract is ever wholly derived from differentiated 

 ectoderm will not be accepted by most embryologists without conclusive 

 evidence. 



The second view is that the formation of the inner layer of the hud 

 is a process of gastrulation, giving rise to entoderm, and that the so- 

 called " gastrulation " of the sexual ontogeny of Phylactolacmata is to be 

 regarded as a precocious ingression of mesoderm only. 



Two considerations are opposed to this view. In Membranipora there 

 is a gastrulation which gives rise to the entoderm and mesoderm of the 

 larva; and since the gastrulation of Phylactolacmata is similar, these 

 elements must be potentially present here also. The "gastrulation" 

 in Bryozoa is a normal one ; if there is any entoderm in the body wall 

 giving rise to the inner layer of the bud, it must have been ento- 

 derm which failed to become invaginated. But what, in the second 

 place, is to be gained by assuming that the inner layer of the bud is 

 formed from entoderm 1 Here is as great a difiiculty as before, since the 

 nervous system originates from this layer. It has been maintained in 

 many cases that the nervous system arises from mesoderm, and Seeliger 

 ('89, p. 602) believes that it is formed from that layer in the non-sexual 

 reproduction of some Tunicates ; but I know of no good evidence of its 

 origin in any of the Triploblastica from entoderm. 



Before going on to state my conception of the significance of the 

 inner polypide layer, I desire to call attention to the conditions in the 

 region at which it is first formed. I have shown above (page G9) that 

 the primary polypide or polypides arise from the pole of ingression in 

 Phylactola?inata, and that therefore in this group the aboral pole (in the 

 sense of Rarrois) corresponds to the pole of ingression. As I under- 

 stand B.UTois, he means l)y oral pole merely the pole which in Cypho- 

 nautes, for instance, bears the mouth, — the pole also by which the 

 larva attaches itself. Brnem ('90, p. 123, foot-note), however, interprets 

 " oral side " in Barrois's sense to mean in the last instance the place at 

 which gastrulation takes place. Perhaps Barrois does somewhere state 

 such to 1)0 the significance of his term (I have not found the place), 

 but in that case I can only say that, to my mind,, he has not produced 

 sufficient evidence to prove that the oral pole of the larva of Gymno- 

 Isemata is the same as the pole of ingression in the gastrula ; nor, in my 

 opinion, has any other investigator done so. Nearly all species studied 

 have a stage early in their development when their poles are very sim- 

 ilar, and orientation certainly would be exceedingly difficult. One of the 



