MUSEUM OF COMPARATIVE ZOOLOGY. 97 



best figured series in which to trace the homology of poles is that shown 

 by RepiachofF ('80, Taf III.) for Bowcrbaukia. So far as the figures 

 go, one would conclude that Figure 10 A and its predecessors were 

 oriented in the opposite direction to Figure 11 and its successors, 

 which would result in placing the pole of iugression (Fig. 9) at the 

 aboral pole of the larva, — the pole which here, as in all other Gymno- 

 laemata, and, I believe, in Phylactolseraata also, gives rise to the primary 

 polypide. I have given above additional evidence for this conclusion, 

 in my argument to prove the homology of the larvse and larval organs in 

 Phylactohemata and Gymnolasmata. 



IVie polyp ides arise in Phylactolcemata at the pole of ingression, which 

 is prohahly homologous with the aboral pole of Gymyiolcemata. The pole 

 of ingression, or the region of the lips of the blastopore, must be regarded 

 as being a region of less pronounced differentiation than the rest of the 

 gastrula. Its cells cannot be said to be either ectodermal or ento- 

 dermal. It is an interesting fact, that it is just these indifferent cells 

 — not yet either ectoderm or entoderm — that give rise to the inner 

 layer of the polypide, from which organs usually considered ectodermal 

 as well as those considered entodermal arise. 



My conclusion, then, the objections to which I fully realize, may be 

 stated in the following words : The inner layer of the polypide bud is 

 composed of cells derived from the rim of the blastopore. Such cells are to be 

 regarded as still indifferent, and as first becoming differentiated into ecto- 

 derm and entoderm in the formation of the young polypnde. 



Just when and where, on this hypothesis, the differentiation into 

 ectoderm and entoderm occurs, is an important question ; but unfor- 

 tunately I cannot answer it decisively. It may be pointed out, however, 

 that it has now been shown for most Ectoprocta that the lining of the 

 middle part of the alimentary tract is formed independently of the 

 oesophagus, and by an actual or potential outpocketing of the primitive 

 simple sac of the bud. In Endoprocta there is a similar outpocketing, 

 which, however, arises in connection with the oesophagus, and is formed 

 independently of the rectum. 



This is perhaps the proper place to call attention to the fact that the 

 mesodermal outer layer of the bud has a very embryonic character at 

 the budding region. This is indicated by tire fact, that in Phylactolaemata 

 (in which group alone I have studied the subject) eggs always arise 

 from that part of the coelomic epithelium which lies in the budding 

 region (cf. Plate XL Fig. 93). In Pyrosoma, also, according to the 

 researches of Seeliger ('89, pp. 598-G02) the mesoderm of the budding 



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