igos.i EXCRETORY ORGANS OF METAZOA. 601 



ontogenetic formation. We shall place first relative position with 

 regard to the outer skin, the blastocoel and coelom, the intestine and 

 the genital organs. These relations involve genetic connections 

 with the particular germ layers, and a word of discussion may be 

 in place with regard to these. The concept of the essential homol- 

 ogy of the primary germ layers has been many times attacked since 

 its formulation by Huxley and Kowalevsky. Yet these objections 

 have been weakened by much of the more recent work. Ectoblast 

 always furnishes nervous elements, entoblast originates digestive 

 and assimilative parts, from the mesoblast come the reproductive 

 cells ; these are cardinal distinctions that seem to hold throughout 

 the Metazoa. Therefore it is no valid objection to the idea of the 

 homology of these layers to cite the observations of Chun on 

 Ctenophores, that in the process of gemmation an ectoblastic out- 

 pushing gives rise to both ectoblast and entoblast. This observa- 

 tion can rather prove only that such an ectoblastic bud is not purely 

 ectoblastic but mixed in its nature. And when Heymon's studies 

 on Insects, resulting in the completely ectoblastic formation of the 

 whole intestine, are brought up as an objection, it may be answered 

 that the observational distinction of the germ layers in insects is 

 very difficult, and also that these conclusions have not been corro- 

 borated by all subsequent examiners. The oft-cited case of the 

 Trematodes, to the effect that the embryo throws off its whole ecto- 

 blast, must now be allowed to drop since Goldschmidt has demon- 

 strated that it is not the true ectoblast but only a follicle cell la>er 

 that becomes so moulted. For these and other reasons those critics 

 are becoming fewer who maintain that ectoblast is not always 

 homologous with ectoblast, and entoblast with entoblast throughout 

 the Metazoa; and the most painstaking of all embryological work, 

 that on cell-lineage, bears out most strongly the well-founded general 

 homologies of these primary layers. The discussion has shifted 

 rather to the significance of the mesoblast, the existence of which 

 was so stoutl}^ denied by Kleinenberg. This long and wearying dis- 

 cussion has brought out the result, first clearly stated by Meyer, 

 that two kinds of mesoblast are to be sharply distinguished, the 

 primary or mesectoblast, and the secondary or mesentoblast. The 



PROG. AMER. PHIL. SOC, XLVII I90 MM, PRINTED J.A.NUARY I4, I9O9. 



