BO 
THE ORGANIC CELL 
Feetilisation. 
The egg of the sea-urchin is admirably adapted for watching 
the process of fusion between it and the sperm. The pheno- 
menon occurring in sea-water, the germinal cells being cast 
out from the parents, one is able to collect eggs and spermatozoa 
separately, bringing them together in suitable vessels containing 
C 
Diagram of the Fertilisation of the Egg. (After Boveri.) 
A. Egg surrounded by spermatozoa ; on the right one has penetrated the egg 
membrane, and is entering the cytoplasm. The egg nucleus is seen in the centre. 
B. Egg-nucleus, with chromatin reticulum on the left ; on the right is seen 
the sperm-nucleus (head of spermatozoon) preceded by its centrosome and 
attraction sphere. C. Egg-nucleus on the left, sperm-nucleus on the right of 
the centre of egg. D. The centrosome has divided, the two attraction spheres 
separate to form the first cleavage-spindle. E. The first cleavage-spindles 
with splitting of the chromosomes. F. Completion of first cleavage, each 
nucleus contains four chromosomes, two from the egg, and two from the sperm. 
N.B. — In the diagram the sperm -chromosomes are shaded ; those from 
the egg -nucleus are black. 
sea-water. The process can be watched under the microscope, 
and eggs killed at the various stages can be sectioned and 
mounted for future detailed examination. In the explanatory 
diagrams, which are after Boveri, A is the egg, surrounded 
by its envelope, and containing a clear nucleus. Around 
