3o6 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



deserve close attention, for they are indeed among the most characteristic events 

 of early development and exercise a definite effect on ultimate growth processes. 



Naturally, in treating an extensive succession of intricate events, I shall be 

 forced to be quite selective. The following account presupposes a rather advanced 

 knowledge of the basic events of development. This condition will permit us to 

 focus our attention on events and results most significant for our present purpose, 

 to treat with brievity classical information, while explaining and illustrating in 

 some detail special aspects insufficiently appreciated or too recent to be well 

 known. 



One of the servitudes of Embryology is that we are rarely able to get complete 

 information by studying only the groups or species in which we are, for various 

 reasons, directly interested. We often have to use results of research performed 

 on distant forms, implicitly assuming that, if data are related to events of general 

 occurrence, they may have a general application. This is the reason why we shall 

 feel justified in wandering from one animal group to another, without treating one 

 particular form with completeness. We suppose that the reader will be sufficiently 

 aware of the special aspects of the various materials, for which he can easily refer 

 to elementary textbooks or more extensive treatises (Schleip, 1928; Huxley and 

 De Beer, 1934; Dalcq, 1938b; Weiss, 1939; Dalcq, 1941b; Raven, 1954; Willier, 

 Weiss and Hamburger, 1955; Ktihn, 1955; Waddington, 1956; GaUien, 1958). 



Therefore, in this chapter, the preparatory events of gametogenesis will be 

 considered first, and with them fertiHzation, which affords a landmark between 

 egg and sperm formation and the unfolding of germinal potentialities. Then, in 

 order to follow the course of development, a distinction will be made, for the sake 

 of analysis, between the intrinsic aspects of organization and the processes of 

 induction. The interest of these will justify a rather extensive treatment, which 

 will throw some light on the nature of differentiation and the onset of growth. 



To my deep regret, it will be often impossible to refer to the promoter of a 

 given piece of research or to the earlier works of an author. Quotations are 

 mostly chosen for their applicability and as a source of reference. 



I. ORGANIZING ASPECTS OF GAMETOGENESIS 



The formation of male and female gametes is usually considered from the point 

 of view of chromosomal transformations, which are, of course, of primary im- 

 portance. But, since we are striving to understand morphogenesis, which mobilises 

 the whole resources of the germ, we mvist also especially draw our attention to 

 the cytoplasm. 



A. The cytoplasm of the gametes 



The existence of the germinal Uneage being taken for granted, we have to 

 remember, however trivial it may be, that the cytoplasm of oogonia or sperma- 

 togonia is continuous with the cytoplasm of primary gonocytes, and, through 

 these, the cytoplasm of the parent egg and of its blastomeres. So far no one can 

 make a thorough inventory of the implications of this continuity. But thanks to 



