No. I.] GERM-LAYERS FN CLEPSIATE. 149 



thing so eminently satisfactory in theory, but without a particle 

 of foundation in fact ! 



Among the figures given by Nusbaum (No. 8), I find only 

 one (Fig. 33, PI. III.) in which there is a median ventral 

 thickening of the epidermal layer that might be mistaken for 

 the basis of the nerve-chain. I have seen the same thing, 

 and my 'first thought about it was that it represented a neural 

 thickening. I shall show that this thickening is a glandular 

 organ, and that the nerve-chain arises beneath it, and never 

 has any connection with it. Nusbaum gives no figures in 

 which such a thickening is seen at any point far behind the 

 anterior ends of the germ-bands. In his Fig. 34, representing 

 a transverse section of some part of the trunk of the embryo, 

 there is not the slightest evidence of an epidermal thickening. 

 On the contrary, the nerve-chain is here sharply marked off 

 from the epidermis, although in contact with it. How does it 

 happen that the nerve-chain is here farther advanced in devel- 

 opment than at the anterior end? We ought, of course, to find 

 the development less and less advanced as we go from the head 

 towards the hind end. The broad neural groove ("breiter Ner- 

 venrinne") shown in his Fig. 34 is purely ideal. Both Bergh 

 and Hoffmann agree with me in affirming that the epidermal 

 layer is here continuous, and not interrupted, as represented by 

 Nusbaum. The epidermis completely covers the neuro-nephric 

 stratum at a very early stage, and even advances more rapidly 

 than the deeper portions of the germ-bands, meeting in the 

 median ventral line, and forming a continuous layer before their 

 junction, as shown in Fig. 7, PI. IV. Both the " neural groove " 

 and the "thin ectodermal folds" are inventions, pure and simple, 

 — products of a fertile imagination, which pays more respect 

 to the supposed requirements of some fascinating theory than to 

 the needs of thorough and accurate observation. 



Nusbaum goes completely astray in his account of the second 

 layer (neuro-nephric) of the germ-bands, as will be seen from the 

 following (No. 22, p. 613): "Die acht grossen Zellen, von 

 Whitman 'Neuroblasten* genannt, die am Hinterende des Embryo 

 friih auftreten und als Producte des primitiven Entoderms aufzu- 

 fassen sind, erleiden folgende Veranderungen. Sie unterliegen 

 einer energischen Theilung und vermehren sich fort. und fort in 

 der Richtung von hinten nach vorn. Auf einem friihen Stadium 



