SECTION II 



Ontogeny, Bionomics, Treatment & Control 



PART I 



CHAPTER I 

 GAMETOGENESIS 



A. C. WALTON, KNOX COLLEGE, GALESBURG, ILL. 



The history of the formation of the germ cells among 

 nematodes is so closely bound up with the processes of 

 meiosis and fertilization that consideration of any one 

 of these phenomena involves a discussion of all three. 



The process of meiosis, or reduction division, was first 

 announced by Van Beneden in 1883 in his report of 

 studies on the egg and spermatozoon of Parascaris 

 equorum (Ascaris megalocephala) and the fact that the 

 gametes contained only one-half the number of chromo- 

 somes found in the body cells, equally divided as to origin 

 from each parent, is one of the most fundamental con- 

 cepts of the fields of Evolution and Heredity. The 

 realization that Parascaris germ cells were large, easily 

 obtained, and very simple in their nuclear organization, 

 led to their use as study material in the rapid advances 

 of Cytology during the last decade of the nineteenth 

 century. 



The germinal cells of nematodes are differentiated 

 during the very early cleavage divisions of the zygote 

 and furnish a very clear history of germ-cell isolation, 

 especially in those forms which show the "diminution" 

 phenomenon. Ignoring for the time this peculiar pro- 

 cess, the mitotic activity of the somatic and of the germinal 

 cells has afforded a fruitful source for cytological in- 

 vestigations. It was from the study of Parascaris 

 (Ascaris megalocephala) that Van Beneden (1883), Boveri 

 (1887, 1888, 1890), Herla (1893), and Zoja (1896) laid 

 the foundation work that established the doctrine of the 

 genetic continuity of chromosomes, not only as to material, 

 but also as to individual size and shape. The same 

 material allowed Boveri (1909), Bonnevie (1908, 1912), 

 and Vejdovsky (1912) to work out the structure of the 

 individual chromosomes; a result that later workers on 

 other materials have largely substantiated as to the 

 main interpretations. (For a review of the literature 

 up to 1923 see Walton, 1924). 



As a result of these and other studies on nematode 

 materials, the process of somatic mitosis seems to fall 

 in line with the general system as follows : The reticulum 

 of the nucleus becomes organized into a number of fine 

 chromidial threads (Brauer, 1893) during the early pro- 

 phase; these undergo an accurate longitudinal splitting; 

 shorten and thicken, and take their places as individual 

 chromosomes in the equatorial plate at the end of the 

 prophase. The metaphase proper is practically absent, 

 as splitting occurs early in the prophase. During the 

 anaphase the chromosomes separate along the line of 

 longitudinal splitting and pass to the two poles of the 

 achromatic spindle. During the telophase each group 

 of chromosomes becomes transformed into a new nuclear 

 reticulum in which the individual chromosomes may lose 

 their visible outlines, but not their actual identity, through 

 vacuolization (Van Beneden, 1883, 1887), branching 

 (Rabl, 1889; Boveri, 1887), or chromonema formation 

 (Vejdovsky, 1912). The somatic numiber of chromosomes 

 remains constant although they are divided equationally 

 at each division and, since they are all descendants of 

 the chromosomes of the zygote nucleus, the chromatic 

 material of every germ cell and of every body cell is directly 

 derived from that whish was brought into the zygote 

 nucleus by the egg and sperm nuclei of the preceding 

 generation; a fact of enormous importance in the study 

 of heredity and development. 



The achromatic as well as the chromatic elements of 

 the cell have been studied carefully in nematode material. 

 Van Beneden (1887) and Boveri (1887) established the 



thesis that the centrosome is a permanent and genetically 

 individual cell structure. Although usually regarded as 

 extra-nuclear in position, it is reported as of intra- 

 nuclear origin in P. equorum var. univalens (Brauer, 

 1893) and in P. e. var. bivalens (Sturdivant, 1931). In 

 spite of much criticism, modern workers in the same 

 field have substantiated this conclusion, at least as to 

 ce'ls of Parascaris equorum (Fogg, 1931; Sturdivant, 

 1934), although the exact nature of the structure is still 

 unknown. The centriole divides (Boveri, 1900; Sturdi- 

 vant, 1934) before any other visible evidence of mitosis 

 appears, and migrates to opposite sides of the nucleus 

 to form the poles of the next spindle figure. The spindle 

 proper (first seen in nematode materials by Auerbach, 

 1874), the mitome ring, and the astral rays appear to be 

 composed of granules and fibers which probably are the 

 result of chemical fixation of what, in the living cells, 

 are delimited currents of nuclear material in reaction 

 with certain cytoplasmic elements which center at the 

 centrosomal points, and are not fibers of actual material 

 identity as stated 'by Boveri (1888). The fibers appear 

 before the nuclear membrane disappears, and their extra- 

 or intra-nuclear origin may depend upon the differential 

 permeability of the membrane; streaming may first begin 

 either in the cytoplasm or the karyoplasm, depending 

 upon the physiological condition of the two substances. 



Cytokinesis, as opposed to karyokinesis, is usually 

 accomplished by a process of constrictive furrowing 

 caused by differential surface tension and surface stream- 

 ing phenomena (Spek, 1918 and 1920 in Rhabditis pellio 

 and R. dolichura) which seem to depend upon the changes 

 in the permeability of the cell membrane. These phe- 

 nomena seem to be correlated with karyokinesis through 

 the medium of the achromatic spindle. 



Meiosis, as a phenomenon, accomplishes the "reduction 

 of chromosomes" in that it affords an opportunity for 

 the numerical reduction of the constant somatic com- 

 plement of chromosomes (the diploid number) to the 

 gametic (haploid number), and also separates the mem- 

 bers of each pair of homologous chromosomes present 

 in the somatic complex. In such a process, two forms of 

 chromosome division occur; (a) separation equationally 

 of split chromosomes, and (b) disjunction of homologous 

 (paired) structures. As in most animals, meiosis occurs 

 in connection with gametogenesis among the nematodes. 

 In the male those cells (spermatogonia) destined to give 

 rise to the spermatozoa undergo a series of ordinary 

 equational divisions until a certain definite number is 

 reached. The last generation of these cells undergoes a 

 growth period during which the homologous male- and 

 female- derived chromosomes are paired. The resultant 

 cells (the primary spermatocytes) have the haploid 

 number of chromosome pairs. Two successive meiotic 

 divisions, one disjunctive and the other equational, follow 

 without complete nuclear reorganization during the in- 

 terphase. The first division gives rise to two secondary 

 spermatocytes and the second divides the two secondary 

 spermatocytes into four spermatids. Normally each 

 spermatid metamorphoses into a spermatozoon, giving 

 four spermatozoa (male gametes) as the end result of 

 the two meiotic divisions. In certain of the free living 

 nematodes Cobb (1925, 1928) reports the intercalation 

 of a number of equational divisions of the spermatid 

 before the ultimate differentiation of the spermatozoa 



205 



