232 THE CHICK. 



taking place. The spermatozoa are received by the hen some 

 time before the laying of the eggs, and retain their vitality 

 and functional activity for about a fortnight. 



THE EARLY STAGES OF DEVELOPMENT. 



1. Segmentation of the Egg. 



Segmentation commences about the time the egg arrives in 

 the lower, thick- walled part of the oviduct, or uterus ; it is con- 

 tinued actively during the stay of the egg in the uterus, and is 

 completed about the time the egg is laid. 



Segmentation, as already noticed, does not concern the whole 

 egg, but is confined to the germinal disc ; and the hen's egg is 

 therefore spoken of as meroblastic, inasmuch as only a portion 

 of it takes part in the process of segmentation, in contradistinc- 

 tion to the holoblastic eggs of Amphioxus and the frog, in 

 which the entire egg is divided by the first cleft into two equal 

 parts. 



In the hen's egg, segmentation commences with the forma- 

 tion of a vertical groove or furrow, which runs across the middle 

 of the germinal disc, but does not quite reach its edge at either 

 end. This is very shortly followed by a second furrow, crossing 

 the first one almost at right angles. Four radial furrows soon 

 appear, about midway between the two first ones ; and then by 

 cross furrows each segment becomes divided into a central and a 

 peripheral portion. Additional furrows soon appear, both radial 

 and concentric, and by these the germinal disc becomes cut up 

 into a mosaic of segments of irregular shape and size, separated 

 from one another by the furrows or grooves (Figs. 102, 103). 



The segmentation is slightly excentric almost from its first 

 commencement, the furrows extending nearer to the edge of 

 the disc, and the segments being smaller, at one side (the lower 

 side of Fig. 103), which corresponds to the future posterior end 

 of the embryo ; while at the opposite, or anterior part, of the 

 germinal disc the furrows stop short further from the edge, and 

 the segments are of larger size. 



Sections of the germinal disc at the stage represented in 

 Fig. 103 show that, in addition to the vertical furrows by which 

 the mosaic pattern is produced on the surface, horizontal clefts 



