154 WHITMAN. ' [Vol. I. 



Balfour based his account of the formation of the germ-layers 

 in Clepsine on the observations presented in my earlier paper 

 (No. i), and offered one or two critical remarks that deserve 

 notice. Referring to my statements on the origin of the nerve- 

 chain from the neuroblasts, he says (No. 12, pp. 288-9) * " Such 

 a mode of origin for a ventral ganglionic chain is, so far as I 

 know, without a parallel in the whole animal kingdom. 

 Till more evidence is brought forward by Whitman or some other 

 observer in support of the view that the so-called neuroblasts 

 have any share in forming the nervous system, they must, in my 

 opinion, be regarded as probably forming, in conjunction with 

 the mesoblasts, two simple mesoblastic bands. Kowalevsky 

 has, moreover, briefly stated that he has satisfied himself that 

 the nervous system in Clepsine originates from the epiblast, — 

 a statement which certainly could not be brought into harmony 

 with Whitman's account." 



In reply to these objections the following considerations 

 have been offered (No. 28, p. 392) : — 



"With reference to Clepsine, Kowalevsky remarks: 'I pre- 

 served only several stages in weak chromic acid, and from sec- 

 tions of these I could only convince myself later of the origin 

 of the nervous system from the upper layer.' This is all he has 

 said on this point; and I will now show that, if we do not go 

 behind the verbal statement itself, it does not even require to be 

 brought into harmony with my account, since it is precisely 

 what I have claimed. The four rows of neuroblasts in each 

 germ-band lie, at the outset, at the surface, and must therefore 

 be considered a part of the epiblast, although a specialized 

 part. It is simply a precocious differentiation of the edge of the 

 epiblast, by which epidermal and neural elements become dis- 

 tinctly marked at an unusually early stage. In the course of 

 the epibolic growth of the ectoderm the epidermal portion pro- 

 gresses somewhat more rapidly towards the lower pole than 

 the germ-bands, and thus sweeps over the neural portion. But 

 it seems to me plainly a matter of little importance whether the 

 neural portion loses its surface position during the epiboly, or 

 immediately after the conclusion of the concrescence of the 



(28.) Whitman, C. O. A Rare Form of the Blastodenn of the Chick, and its 

 Bearing on the Question of the Formation of the Vertebrate Embryo. 

 Quart. Journ. Mic. Sc, XXIII. 1883. 



