879 



developed, — grows downwards to the lateral surface of the body to 

 the level of the iiotochord : a portion of it then fuses with the epiblast, 

 which is thickened at this point. It may be remembered that I de- 

 scribed in Elasmobranchs the origin of the ganglion from such an 

 epiblastic thickening. This view I see good reason to slightly modify 

 in the sense that such thickenings are seats of origin of only por- 

 tions — but very large portions — of the cranial ganglia. In the 

 Chick, and in all strictly air breathing Vertebrates in which the gill 

 sense organs are absent as functional organs, the thickening above re- 

 ferred to, though morphologically in all cases to be regarded as the 

 rudiment of the primitive branchial sense organ is concerned iu the 

 formation of certain parts of the particular cranial , ganglion. I hold 

 that after the fusion a certain number of cells are given ofi' from the 

 thickening into the ganglionic Anlage, and that after the ganglion 

 separates from the epiblast the modified sense epithelium usually dis- 

 appears^). Whether as in Elasmobranchs the thickening is also a point 

 of origin and further development of sensory nerves from the skin 

 is one on which at present I cannot be absolutely certain, but I believe 

 such to be the case. 



The spinal ganglia in the Chick. 



The Chick is so readily accessible and is so much used for edu- 

 cational purposes that I need hardly apologize for giving the de- 

 velopment of the spinal ganglia in detail for this form. The ease 

 with which one can obtain all stages of development renders this ani- 

 mal invaluable for filling in the links of a comparative embryological 

 research. 



As in the case of the cranial ganglia one meets with the first 

 traces in very young embryos — nay, so young in some cases that 

 not even a single somite or protovertebra is yet formed 2). On the 

 whole the best embryos for the first traces are those with 4—6 so- 

 mites. I need hardly say that in such embryos the neural canal 

 is still quite open. 



No observer till now has recorded any appearance of ganglia 

 until the neural canal closes; and nearly all agree that the ganglia 

 arise as outgrowths of the spinal cord. 



1) I hope soon to shew that the end organs of taste arise from 

 such thickenings which have wandered through certain gill clefts into the 

 buccal cavity. 



2) I can also find traces of spinal ganglia Anlagen in a Lizard em- 

 bryo in which only one somite is yet developed. 



