EMBRYOLOGY AND REPRODUCTION. 75 



An eggj it will be remembered, usually contains 

 yolk, which, supplies food to the growing animal. The 

 stages spoken of above on page 70 are described with- 

 out reference to the yolk. When much yolk exists, 

 the embryo which corresponds to the gastrula may 

 be considerably modified. In the hen's g^%, for in- 

 stance, it lies flat upon the yolk, so that its compo- 

 nent layers are nearly parallel, the top layer repre- 

 senting the outside layer of the bag-like gastrula, whilei 

 the lower layer, which rests upon the yolk, represents 

 the inner layer of the gastrula (hypoblast) . Eggs which 

 have little or no yolk are called Holoblastic, because 

 the whole of the e^^ takes a direct part in the forma- 

 tion of the embryo, and eggs with much yolk are called 

 Meroblastic, because only part does so. It is so 

 easy to procure hen's eggs, and watch their different 

 stages of development, that the student of e^^ deve- 

 lopment is always introduced to " the chick " for a 

 first lesson. The outside of the egg demands a little 

 attention. The protective shell of the &^^ consists of 

 carbonate of lime ; if the hens are not supplied with 

 food that contains plenty of this, they will lay eggs 

 with soft shells. The white serves more than one 

 purpose : not only is it absorbed by the chick, but it 

 acts as a cushion to keep it from being killed by 

 knocking against the shell, if the q^^ is jarred in its 

 early stages. The germ, or embryo, lies like a flat 

 cake on the top of the yolk. When it is cut through 

 in a direction perpendicular to its flat extension, in 

 slices thin enouorh to be examined under the micro- 



