Section V 

 EMBRYOGENESIS: PREPARATORY PHASES 



CHAPTER 1 



Gametogenesis, Fertilization and 

 Parthenogenesis 



ALBERT TYLER 



GAMETOGENESIS 



ORIGIN OF THE GERM CELLS 



An understanding of the factors that endow 

 egg and spermatozoon with the ability to 

 unite and produce a new individual may be 

 expected to depend largely on knowledge of 

 the manner of origin and formation of the 

 gametes. While such knowledge is very far 

 from complete there is a considerable body of 

 descriptive and experimental information re- 

 lating to it. To illustrate the nature of this 

 information and some of the problems in- 

 volved certain of the pertinent investigations 

 will be briefly reviewed here. Questions con- 

 cerning the determination of the gonad are 

 discussed in another chapter of this book. 



Much of the stimulus for work on the 

 origin of the germ cells was undoubtedly 

 provided by Weismann's (1883, 1893) views 

 concerning the continuity and segregation of 

 the germ plasm. His concept of a distinct 

 germ plasm is sufficiently familiar, even to 

 beginning biologists, so that it does not re- 

 quire detailed presentation here. A critical 

 review of the original basis of the concept 

 has been published recently by Berrill and 

 Liu ('48). 



In many species of animals the primordial 

 germ cells may be recognized at an early 

 stage of development and, particularly 

 among the invertebrates, there are cases in 

 which a "germ-line" is manifest from the 

 start of cleavage (see Wilson, '25, and Boun- 

 oure, '39, for references and brief accounts). 

 In these the identification of the primordial 

 germ cells is based on certain distinguishing 



features of the nucleus or the cytoplasm. We 

 shall consider some examples briefly here. 



Chromatin Diminution in Ascaris. The clas- 

 sic case is that of Ascaris megalocephala 

 (Parascaris equorum) in which Boveri (1887- 

 1910) described a process, termed chromatin 

 diminution (Herla, 1895), occurring during 

 cleavage in the cells that are to form somatic 

 tissue but not in those that are to form the 

 germ cells. In this process the mid-portion 

 of each of the chromosomes (two in A. 

 megalocephala univalens) breaks up into a 

 number of small fragments (about 10 per 

 chromosome according to Fogg, '30), leaving 

 two large terminal pieces. The smaller frag- 

 ments reconstitute the daughter nuclei and 

 continue regular mitotic division but the 

 terminal pieces remain in the cytoplasm and 

 slowly disappear. The ability of the small 

 fragments to continue mitotic division is evi- 

 dently due to the fact that each possesses a 

 centromere, the original chromosomes being 

 polycentric, as Schrader ('35) and White 

 ('36) have pointed out. It is now well known 

 that chromosomal fragments devoid of a 

 centromere fail to attach to the spindle and 

 to divide. 



The diminution phenomenon starts at the 

 second or the third division of the egg. In 

 the former case it occurs in the dorsal, so- 

 matic cell (Si) and not in the ventral, stem 

 cell (Pi), as illustrated in Figure 45. In the 

 latter case it occurs in three cells at once 

 but not in the stem cell P2. In both cases 

 P2 divides into a somatic cell which under- 

 goes diminution (at its next division) and 

 another stem cell, P3. The latter again di- 



170 



