16 



THE INITIATION OF DEVELOPMENT: 



of pronuclei (Fig. 4, ^). They meet, forming a so-called amphi- 

 aster. Then the nuclear membranes of both pronuclei disappear, 

 thereby liberating their chromosomes. A mitotic spindle de- 

 velops between the poles of the amphiaster, and the chromosomes 



Fig. 4. Cytology of fertilisation in a sea urchin, 1-4: formation of 

 the sperm aster, fusion of the nuclei of egg and sperm; 5-7: forma- 

 tion of the amphiaster, first cleavage division, u : female pronucleus; 

 sp: sperm nucleus. After Fry and Parks. 



arrange themselves in the centre of the spindle (Fig. 4, ^■'^), 

 thereby completing the union of the two nuclei (amphimixis). 

 The next step will be a cell division, the first cleavage^ which 

 marks the beginning of the development of the germ. 



A number of experiments have provided information as to 

 the factors governing the processes here described. First, it 

 appears that at least two conditions must be fulfilled for the 

 development of the sperm within the Qgg to take place: (1), 

 the maturation of the egg must have begun, so that the nuclear 

 sap of the germinal vesicle has mixed with the egg cytoplasm 

 (p. 10); (2), the sperm must have traversed the egg cortex. 

 Sperms that have been artificially injected into the egg remain 

 completely inactive (Kite). The same is true of sperms which 

 have penetrated into egg fragments without cortical plasm 

 (Chambers). Evidently, the sperms are activated during their 



