2IO 



THE EVOLUTION OF MAN. 



Fig. 36. — Monerula of a Mammal (Eabbit). The fertilized egg- cell after 

 loss of the germ-vesicle is a simple ball of protoplasm (d). The outer 

 envelope of this is formed by the modified zona pellucida (z) and by a mucous 

 layer (h), which is deposited on the outside of the zona. A few sperm-cells 

 are still visible in this mucous layer (s). 



Fig. 37. — Parent-cell, or cytula, of a Mammal (Eabbit) : k, parent-kernel, 

 or nucleus ; n, nucleolus ; p, protoplasm of the parent-cell ; z, modified 

 zona pellucida ; h, external albuminous envelope ; s, sperm-cells. 



Fig. 38. — Commencement of cleavage in the mammalian egg (Rabbit). 



The parent-cell has separated into two differing cells ; the brighter, 

 mother-cell of the skin-layer (e), and the darker, mother-cell of the in- 

 testinal layer (i) : z, zona pellucida; h, external albuminous envelope; 

 s, dead sperm-cells. 



