1887.] 359 [Jeffries. 



I held that there was an outside single layer of cells distinct from 

 those below ; my reasons being the early formation of such a 

 superficial layer in the chick and its endurance till near the end of 

 incubation, the presence of the same structure in the other verte- 

 brate groups, its renewal with each moult in reptiles and appar- 

 ently in the pin feathers of birds. 



To this Dr. Gardiner takes exception on grounds which it is 

 hard to summarize from his article. 



First, he considers there cannot be enough growth in this layer 

 to cover the chick, that cell-division cannot be sufficient, and 

 that the increase in size is surely not enough. In proof of this 

 he states that he has measured camera drawings of the epitrichium 

 given by me, of the five and twenty days' chick and finds the cells 

 about twice as large in the latter, as in the former. 



Second, that gaps occur in this outer layer which are stopped 

 by cells from below. 



Third, that the layer does not exist on the body proper. 



It will be noted that objections one and two presuppose the ex- 

 istence of an outside layer, otherwise he could not talk about the 

 cells below becoming incorporated in it. But the existence of 

 such a layer is put bej'ond doubt by the specimens on the table. 

 As to cell-division he does not den}' it, and it undoubtedly does 

 occur while the amniotic fluid maintains its original constitution 

 and affords a good food supply. This division is shown by the 

 double nuclei found by all. In regard to the size of the cells, Dr. 

 Gardiner has made some serious mistakes due perhaps to haste. 

 I do not give a figure of the epitrichium of a twenty days' chick, 

 so he could not measure it as he maintains. The figure of the four- 

 teen days' chick shows an increase of about two (really two and a 

 half) in linear measure making six and a fraction times as much 

 surface area. I hold that the cell multiplication in the early 

 stages and the cell-flattening with a resultant six-fold surface, suf- 

 fice to cover the chick. As has been shown, Dr. Gardiner's ob- 

 jections are based on errors. 



The second objection that gaps occur which are filled up from 

 cells below is supported by no evidence. He does not describe 

 how the cells below change their form and structure under reaction, 

 nor does he give a figure of such cells in process of transition. 

 Indeed, he nowhere gives a figure showing the epitrichium in a 



