I06 whitman: [Vol. I. 



fusion in the identification of the germ-layers, and to most 

 contradictory interpretations of the strata composing the germ- 

 bands. The association of neuroblastic with mesoblastic ele- 

 ments in the germ-bands, as maintained in my first paper on 

 this subject, proved a serious stumbling-block to Balfour. 



The germ-bands in a closely related group of annelids, the 

 Oligochaeta, were held to be purely mesoblastic, and my sug- 

 gestion that they contained a neural stratum appeared to 

 stand in plain contradiction with well-established views as to 

 the origin of the nervous system. Moreover, Kowalevsky, 

 whose brilliant success in extending the germ-layer theory to 

 the invertebrates had rendered his authority preeminent, had 

 stated, as a fact settled by his own observations, that the ner- 

 vous system of Clepsine, like that of Lumbricus and other 

 oligochaetous annelids, was derived from the " upper layer," i.e., 

 the epidermal layer. This statement, although based upon an 

 evidently hasty and unreliable examination of a few poorly pre- 

 served eggs of Clepsine, was corroborated by more extended 

 studies on Rhynchelmis (Euaxes) and Lumbricus, and later, 

 by the researches of Hatschek, Kleinenberg, and others. The 

 more trustworthy statements of Metschnikoff, published in the 

 same year with Kowalevsky's " Embryological Studies on 

 Worms and Arthropods," escaped the attention of Balfour, and 

 thus the testimony of numerous excellent observers as well as 

 theoretical considerations appeared to stand in the way of ac- 

 cepting my conclusions. The contradictory results since reached 

 by Bergh and Nusbaum have only made it still more desirable to 

 reexamine the subject with greater care and thoroughness. 



Bergh's important researches on the development of the 

 Gnathobdellidse have led him to dispute the concurrent testi- 

 mony of previous investigators on the origin of the epidermal 

 layer in Clepsine ; and thus we are left in a state of uncertainty 

 regarding the origin and limitations of the germ-layers, not very 

 far removed from that in which Hoffmann found himself, when, 

 at the end of his second memoir on this subject, he frankly con- 

 fessed his inability to distinguish " Keimblatter " in the Hiru- 

 dinea. 



Respecting the histological differentiation of the germ-band 

 strata, there is still less unanimity of opinion. Even the latest 

 writers, Bergh and Nusbaum, have failed to agree on the 



