16 Hyhridizaiion of Echinoids. 



during the time extending from the instant of contact of the sperma- 

 tozoon with the surface of the egg to the completion of fusion of the 

 germ nuclei, although we have tacitly admitted the logic of the 

 position that fertilization is not completed until the conjugation of 

 homologous maternal and paternal chromosomes. In practice, 

 however, the word fertilization is often used as though it applied 

 merely to the initial impulse to development. 



Strictly speaking, the development of a uniparental individual 

 begins with the separation from the primordial germ-plasm of the 

 material which is to form the ovum from which the body of that 

 individual is to be differentiated. All of the so-called stages in 

 oogenesis are stages in the development of the individual. In 

 biparental individuals there must be added the processes of formation 

 of the spermatozoon which conjugates with the egg. Development 

 may be slowed down, possibly even suspended, pending the activation 

 of the egg as brought about by the earlier stages of union of the two 

 germ-cells. Development then goes on as a continuous series of 

 reactions, up to a certain point, difficult to ascertain but nevertheless 

 actual, when the progressive differentiations in both body and mind, 

 if one were to consider higher animals, come to an end. Develop- 

 ment may be regarded as completed when progressive differentiation 

 ceases and regressive changes set in. 



The Spermatozoon and Fertilization. 



For purposes of convenience in study and analysis, development 

 is often considered as a series of definitely limited, consecutive 

 stages rather than as a continuous process. Indeed, the course of 

 development seems to fall rather naturally into periods, the beginning 

 of each of these standing out as a critical point in the life of the 

 organism. One of these periods begins with the activation of the 

 egg to its development as a multicellular organism. 



Lilhe (1919, p. 129) says concerning fertilization: "It is a series 

 of reactions which can not be regarded as complete until full capacity 

 for development and inheritance is attained by the zygote." In his 

 discussion of the physiology of the spermatozoon, Lillie distinguishes 

 between cortical block to the fertilization reaction and internal 

 block to later stages in fertilization. From my study of hybrid 

 material I should like to go a little further in the characterization 

 of stages, defining cortical block, as Lillie has done, as the block to 

 the cortical reaction, but limiting the use of the term internal block 

 to the period during which conditions which prevent the union of 

 sperm and egg nuclei are effective, and adding developmental block 

 as a designation of the block that may become evident after a success- 

 ful activation of the egg and the union of the two germ nuclei. This 

 block to development maj' operate at an earlier or a later period, 



