LECTURES ON EMBRYOLOGY. 



89 



Polypi is shown when examining their embryolo- 

 gy. Here (Plate XLVIII) are changes in one of 



[PLATE XLVIII CHANGES OF THE BRYOZOA J 



these Bryozoa, which have been investigated by 

 Professor Van Beneden, (Fig. A). The bud-like 

 egg which arises from the main cavity does not 

 produce a terminal germ, from the lower centre 

 of which the main cavity proceeds, but produces 

 (Figs. D, G) a division of this yolk-like mass, un- 

 dergoing all the processes of division which we 

 have elsewhere observed, and finally assuming an 

 elongated form. From the beginning it exhibits 

 the peculiar character of Mollusks, which distin- 

 guishes them from Radiata. Their bilateral form, 

 on the longitudinal axis, is observed in these 

 germs. And thus, going on further, the margin 

 becomes serrated, (see Fig. E), the internal cavity 

 growing deeper and deeper, introducing the whole 

 mass of yolk within, (Figs. H, K) with appendages 

 above. These appendages will soon open, and 

 you will have (Plate XLVIII, fig. C) a large alimen- 

 tary canal, with a central cavity placed in a dis- 

 tinct cavity of the body, with tentacles round the 

 opening ; so that this structure is distinct from that 

 of the Radiata. 



But I must dissent from the conclusions which 

 Professor Van Beneden has deduced from his ob- 

 servations. From the manner in which the yolk 

 is placed in the interior of the alimentary canal, he 

 concluded that there is no difference between the 

 Radiata and the Mollusca in their embryonic 

 growth, as the yolk is formed around the cavity, 



and aa the yolk is introduced from the lower side 

 in both. But he overlooks that in Radiata the 

 centre of development is really the centre of the 

 mass, and that the further growth takes place in 

 all directions simultaneously, by a uniform, all- 

 sided development; whilst in Mollusks there is 

 from the earliest period this bilateral and longitu- 

 dinalaxis. We might just as well say that the 

 Vertebrated Animals do not differ from the Radi 

 ata, because in the former the yol k also is introduced 

 from the lower side into the animal. But we have 

 here another difference among Vertebrates. Be- 

 sides the lower cavity, there is an upper one form- 



% 



ed ; and so we must admit that the type of Mollash 

 is a distinct type from Radiata, and not to be uni- 

 ted with them 



Professor Van Beneden being one who has 

 traced these investigations extensively, and who 

 has tried to characterize the leading groups of 

 animals upon the first changes of the embryo, I 

 thought it proper to make these remarks now. 



[PLATE LV SYMBOLICAL FORMULA or MOL- 

 LUSCA I 



In order to show more fully the distinguishing 

 characters of Mollusca, compared with the other 

 departments of the animal kingdom, I think it 

 useful to mention here the symbols which I shall 

 use to designate in future that type. la accord- 

 ance with the mode of development of the germ 

 in Cephalopoda, a narrow crescent, placed verti- 

 cally, (PI. LV, fig. A) would give the best image of 

 these animals. and contrast their growth with that of 

 Radiata, which are represented by a horizontal 

 circle. Let this crescent be closed, it may repre- 

 sent the Acephala, (Fig B,) with outward turned 

 margins the Gasteropoda, (Fig. C,) in allusion to 

 the wide gape of the lobes of the mantle, and with 

 a transverse division, (Fig. D,) the Cephalopoda, 

 alluding to the complete separation of the head. 



The propriety of admitting this sign, a closed 

 crescent, to represent the type of Mollusca, will ap- 

 pear much stronger, when I mention that among 

 Cephalopoda the yolk is not entirely transformed 

 into a germ, as it is among the Bryozoa, Acephala 

 and Gasteropoda, only a part of the 3 T olk being 

 modified, so as to form a germ around the yolk. 

 The yolk remains for a great part unchanged, and 

 enters the lower side of the embryo into the ab- 

 dominal cavity, (PI, XXXIX, figs. D, E ) So that 

 we have really in outlines the form of the embry- 

 onic sign, which I would preserve for the type of 

 the Mollusca. 



Among the Bryozoa there is a genus called Pe* 

 dicellina, which is minute, and has a still more 

 regular form than the Eschara and Retepora, rest- 

 ing isolated upon small stem?, -with fringes all 



