So-called Mesectoderm in Petromyzon. 



459 



above the yolk, more or less conspicuous groups of cells are proliferated from 

 the ectoderm, at about the level of the chorda and uninterruptedly from the 

 eye backwards along the whole extent of the branchial tract of the enteric 

 canal, and these cells push their way ventrally between the ectoderm and 

 mesoderm. The cells of this first phase are, the author believes, identical . 

 with those of the branchiodermis of Kupffer. 



In quite young embryos the production of these cells goes on throughout 

 one continuous streak, but in a little more advanced stage it is concentrated 

 in certain centres, which Schalk believes to have been detected by him and 

 which resemble the nerve placodes of Kupffer. This concentration indicates 

 the beginning of the second phase of cell production. 



In the second phase of the formation of mesectoderm which Schalk 

 describes, each centre of cell production is found close behind each visceral 

 pouch, except the hyomandibular, which is destitute of such a centre. The 

 statements are illustrated by his text-figs. 16 and 17. The centres are 

 produced by local thickenings of the ectoderm, which appear from before 

 backwards one after another and proliferate the cells in a continuous layer, 

 which becomes pushed backwards so as to be mixed with those of the 

 branchiodermis, so that the cells of both lots can no longer be distinguished 

 one from the other. But he says nothing definite as to whether the 

 branchiodermis, or the cells directly descended from the placodes, represent 

 the formative elements for the branchial cartilage bars. He says merely 

 quite indefinitely : " Wenn nun auch jene kleinen Plakoden mbglicherweise 

 an der Bildung der Branchialnerven Anteil haben, so glaube ich doch 

 behaupten zu konnen, dass ein Teil der aus jenen Epidermisplakoden 

 auswandernden Zellen bei der Bildung der Kiemenknorpel verwandt werden " 

 (14, p. 55). 



Schalk is correct in so far as he excludes the medullary cells from the 

 origin of the mesectoderm ; in other respects the results given by him do not 

 add anything to what everybody had assumed. 



I have observed all the occurrences which Schalk gives in his figures and 

 found that they have nothing to do with the mesectoderm. 



How and when the cell proliferation of the placodes in the branchial region 

 is closed, we cannot learn from the statements given by him at all. But his 

 text-fig. 18 shows a great resemblance to a section from a series of frontal 

 sections through an embryo, 12 to 13 days old, in my possession, which is 

 just hatched or is about to break the chorion. At such a stage as this the 

 mesectoderm is, of course, already fully established and has commenced even 

 to be differentiated, as is clearly seen in the figure referred to ; the con- 

 tinuous epithelium is divided by the outgrowth of the visceral pouches into 



