BARE POIiM OF THE BLASTODERM OF THE CHICK. 393 



has even gone so far as to doubt the accuracy of my observations on Clepsine, 

 on the strength of theoretical convictions, it seems necessary to give it a 

 moment's consideration. Balfour remarks : " Till more evidence is brought 

 forward by Whitman or some other observer in support of the vievr 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 origi- 

 nates from the epiblast — a statement which certainly could not be brought into 

 harm.ony with Whitman's account " (No. S, vol. i, p. 289). With reference 

 to Clepsine, Kowalevsky remarks : " I preserved only several stages in weak 

 chromic acid, and from sections of these 1 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 specialised part. It 

 is simply a precocious differentiation of the edge of the epiblast, by which epi- 

 dermal and neural elements become distinctly marked at an unusually early 

 stage. In the course of the epibolic growth of the ectoderm the epidermal 

 portion progresses 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 looses its sur- 

 face position during the epiboly, or immediately after the concluson of the 

 concrescence of the germ-bands ; and 1 confess that I do not see wherein this 

 view requires " any special support." At the time Balfour penned the above 

 criticism, he evidently was not aware that my observations on the origin of the 

 nervous system in Clepsine were but little more than a corroboration of those 

 of an eminent Russian embryologist. I take this opportunity to express my 

 regret for the same oversight on my own part. It was Professor Metschni- 

 koff who first determined the precise origin of the nervous system of Clepsine.* 

 The accord between Metschnikoff and myself extends not only to the facts 

 observed, but also to the interpretation of these facts. The chief distinction 

 between Clepsine and other Articulata and Vertebrata with respect to the germ- 

 lamellae lies, according to Metschnikoff, in the single fact that " the epidermal 

 layer separates very early from the basis of the nerve system." The associa- 

 tion of neuroblastic with mesoblastic elements in the germ-bands is then a 

 feature which presents no serious difficulty to the comparison before instituted 

 between Chajtopods and Leeches. 



' Kowalevsky, 'Embryol. Stud, an Wiirmeru u. Arthropoden,' p. 3. 

 2 Metschnikoff, ' Beitriige zur Entw'gesch. einiger nied. Thiere.' Melanges 

 Biologiques du Bull, de I'Acad. imp. des. Sci. St. Petersbourg,' vii, 6, 1871. 



