SOME INTERESTING POINTS 



IN CONNECTION WITH THE DEVELOPMENT 



OF THE CHICK. 



BY 



ERNEST CLARKE, M.D., B.S. 

 2(ith MA y, 1886. 



On the surface of that part of the yolk which is uppermost 

 of a fresh fertile hen's egg, is seen a small spot clear in the 

 centre {area pellucida), surrounded by an opaque rim [area 

 opaca). This spot is called the Blastoderm, and it is from 

 this that the chick is developed. 



As soon as the ovum has left the ovary of the hen, if 

 it becomes fertilized by the male element, the process of 

 segmentation begins in it even while it is passing down the 

 oviduct. This segmentation is the gradual formation of 

 cells, and by the time the egg is laid, two rows of these cells, 

 and as soon as incubation begins three rows are seen at the 

 blastodermic spot. It is these three rows of cells that 

 become differentiated, and as we shall see in time, form the 

 complicated structure called the chick. 



First of all, a groove appears called the medullary 

 groove, and the cells grow up on either side of this groove, 

 fold over and enclose it, changing the groove into a canal, 

 called the neural canal, and its length marks the length of 

 the embryo. If we examine a specimen at this early date, 

 we can distinguish three layers of cells, (the upper layer 

 is called the epiblast, the middle layer the mesoblast, and 

 the lower layer the hypoblast), the neural canal, the noto- 

 chord beneath it, and the protovertebras on each side. At 

 each end of the embryo now appears a fold. A head fold and 

 a tail fold, this is the commencement of the gradual separa- 

 tion of the embryo from the egg. 



