MORPHOLOGY OF DEVELOPMENT 



19 



until finally the contents of all the drops have run together 

 into a continuous layer around the egg 1 (Fig. 2). Hence the 

 surface lamellae of the tiny droplets form later the fertilization 

 membrane. 



At higher temperatures the process of membrane formation 

 in freshly removed eggs usually proceeds so quickly that the 

 stages depicted in Figs. 3 to 7 are not distinctly seen and the egg 

 passes directly from the condition of Fig. 1 to that of Fig. 2. 

 At first the membrane adheres closely to the egg, and then of a 

 sudden the space between the cytoplasm and the membrane 

 increases enormously. In this latter way the process of mem- 

 brane formation occurs in the egg of Arbacia. 



After the formation of this fertilization membrane a 

 second change takes place in the surface of the egg, inasmuch as 

 a gelatinous film or membrane (G.M. Fig. 8) gradually appears 

 on the surface of the cytoplasm. This 

 gelatinous film does not form as sud- 

 denly or quickly as the fertilization 

 membrane, but only begins to appear 

 after ten minutes or more. The writer 

 considers it possible that the formation 

 of this film depends upon processes of 

 oxidations, since its formation is de- 

 layed if the oxidations in the egg are 

 retarded by the presence of KCN. 



After membrane formation, the 

 chemical processes which underlie de- 

 velopment set in in the egg. The nuclear material grows and 

 nuclear division occurs; this nuclear division, or rather the 

 so-called nuclear spindle, is visible in the egg which we have 

 chosen for description, viz., that of Strongylocentrotus purpuratus 

 (Fig. 9). The spindle formation visible in this figure is imme- 

 diately followed by cleavage or cell division, i.e., the partition 



1 So-called "perivitelline space.*** 



FIG. 8. Formation of a 

 gelatinous film G.M. around 

 the protoplasm of the egg, 

 about 15 minutes after the 

 formation of the fertilization 

 membrane B.M. 



