ally anterior end of the spermatozoon. It is perhaps 

 to be expected that in Parascaris, and in related genera 

 where chromosomal behavior as well as other criteria 

 point to a high degree of specialization, the acrosome 

 likewise would tend to vary from the normal, as perhaps 

 is shown by its unusual position behind the nucleus. Some 

 nematodes show distinct polymorphism in sperm size, a 

 condition believed to be correlated with the difference in 

 chromosomal numbers between the "male-producing" and 

 the "female-producing" male gametes (Goodrich, 1916; 

 Meves, 1903; JVIulsow, 191il). This chromosome variation 

 is most clearly demonstrable in species in which there is 

 a complex type of "X" chromosome, often involving a 

 large number of chromatin elements (Walton, 1924). 



The completion of the g'erm cycle involves the process 

 of syngamy by which the union of gamete nuclei and the 

 restoration of the diploid number of chromosomes is 

 accomplished. Syngamy in nematodes is complicated by 

 the fact that the maturation of the egg and fertilization 

 proceed simultaneously, the spermatozoon frequently 

 entering the egg during the prophase of the first meiotic 

 division. 



The entire spermatozoon, at least among those that are 

 amoeboid in form, enters the egg and immediately a thick 

 fertilization membrane forms, appearing first near the 

 point of entrance and finally enclosing the entire egg. 

 The reticulated male pronucleus gradually forms from 

 the condensed spermatozoon nucleus, the mitochondrial 

 elements slowly fade into the egg cytoplasm as the male 

 cell wall disappears, and the remnant of the mass of 

 acrosomal material eventually loses its separate identity. 



In the case of many nematodes (Rhabdias bufonis, R. 

 ranae, Rhabditis terricola, Syphacia obvelata (Oxyuris 

 obvelata), Turbatrix aceti) the shell membrane is re- 

 ported as being applied to the egg before the entrance of 

 the spermatozoon!. In such cases a micropylar opening has 

 been described, usually at the end of the egg which was 

 originally attached to the rhachis, and opposite to the 

 pole at which the polar cells are normally extruded. No 

 such structure is necessary in the forms in which sperm 

 penetration precedes egg-shell formation. A structure 

 resembling a micropyle has been described in forms 

 which have the egg shell formed after sperm entrance 

 has occurred. In Ascaridia galli (A. lineata) Ackert 

 (1931) has shown that this is not a true micropyle, and 

 perhaps similar micro-dissection studies might necessitate 

 the revision of the descriptions of the presence of a 

 micropyle in several forms. If a true micropyle is present, 

 it seems obvious that the sperm entrance is fixed at what 

 may be regarded as the vegetative pole of the egg, since 

 many observers have determined that the first polar cell 

 is eliminated at a point opposite the entrance path of the 

 spermatozoon. Probably the same statement holds for 

 those forms in which sperm entrance precedes shell 

 formation, inasmuch as sperm entrance and first polar 

 cell positions are directly opposite in most nematode eggs, 

 and the point of sperm entrance in Parascaris equorum 

 has been shown by Schleip (1924) to be at the originally 

 attached end. This problem is tied up with that of the 



polarity of the egg - which is discussed elsewhere. 



The two pronuclei come to lie side by side, the first 

 cleavage spindle is established, and division follows. 

 During this process the male and female chromosomes 

 occupy opposite sides of the spindle and it is not until 

 the second cleavage division that the two sets of chromo- 

 somes are indistinguishably mixed, although in some 

 cases complete intermingling may be delayed until later 

 in the cleavage phenomenon. 



The development of the egg without fertilization (true 

 parthenogenesis) is rare among nematodes although two 

 species of Rhabditis (Belar, 1923) have been described as 

 showing only a single maturation division and no re- 

 duction in the chromosome number. Kriiger (1913) re- 

 ports that the hermaphroditic Rhabditis aberrans (prob- 

 ably a variety of R. aspera) produces eggs that are 

 apparently parthenogenetic of the diploid type (one polar 

 cell and no chromosome reduction of the somatic number 

 of 18) although frequently the sperm actually enters 

 the egg but degenerates and fails to enter the cleavage 

 nucleus. In a normally dioecious Rhabditis pellio culture, 

 P. Hertwig (1920) found a mutant which produced only 

 one polar ceil without reduction, and thus retained the 

 diploid number (14). None of these eggs would develop 

 unless entered by a sperm, but again in no case did the 

 sperm contribute to the cleavage nucleus. These two 

 cases bridge the gap between normal fertilization and 

 normal parthenogenesis. 



Many nematode species show a "diminution" phenom- 

 enon (Walton, 1918, 1924) in the non "stem-cells" of 

 early cleavage, examples occurring from the second to 

 the sixth division, and then ceasing, as by the sixty-four- 

 cell stage the primordial germ cells are entirely differ- 

 entiated. The process of "diminution" which involves 

 the elimination of a portion of each of the chromosomes 

 in the nucleus is best known in the embryonic cells of 

 Parascaris equorum. In this form the process may begin 

 in the second cleavage of the soma cells although it usually 

 first appears in the third 'cleavage, and then is found 

 in the division of each new soma cell separated from the 

 "stem" cell until the "germ line" cells are definitely 

 isolated. In P. equorum this process is completed during 

 the fifth cleavage. All germ cells retain the undiminished 

 amount of chromatin, while all soma cells have the 

 reduced amount as the result of "diminution". During 

 the prophase of the "diminution division" the chromosomes 

 of the soma cell break up, the center forming a definite 

 number of small chromosomes and the ends several blobs 

 of material. The small chromosomes divide equationally 

 while the larger masses are left behind. The daughter 

 nuclei reorganize without the extruded remnants, which 

 then ultimately degenerate and disappear. The process 

 is quite similiar in other species of nematodes (Meyer, 

 1895; Bonnevie, 1901; Walton, 1918, 1924) except that 

 there is frequently no increase in number of chromosomes 

 during the process inasmuch as the gametic chromosomes 

 in many species are not as complex as they are in 

 Parascaris spp. 



t The formation of a shell before fertilization is dubious. See 

 Sect. 1. Part 3, Chapter 12. B. G. C. 



Fig. 149. 



Gametogenesis. A Parascaris equorum; Prophase of Ilnd. 



oocyte (1st. polar body and "dyad" formation I. B. — Parascaris 

 equorum; Metaphase of Ilnd. oocyte. C. — Parascaris equorum; 

 Anaphase of Ilnd. oocyte (1st. polar body). D. — Parascaris equorum; 

 Telophase of Ilnd. oocyte (1st. polar body and "monad" formation). 

 E. — Parascaris equorum; Formation of pronuclei (1st. and 2nd. 

 polar bodies). F. — Parascaris equorum; Ovum (two pronuclei, cen- 

 trosome dividing, egg membranes omitted). G. — Parascaris equorum ; 

 Ovum (pronuclei approaching, centrosomes at poles, egg mem- 

 branes omitted). H. — Parascaris equorum; Ovum (pronuclei fusing, 

 discrete chromosomes, egg membranes omitted. I — Parascaris 

 equorum ; Prophase of 1st. cleavage spindle. J. — Parascaris equor- 

 um ; Polar view of metaphase of 1st. cleavage spindle. K. — 

 Parascaris equorum; Polar view of metaphase of 1st. cleavage 

 spindle (detached heterochromosomes). L. — Parascaris equorum; 

 Metaphase of 1st. cleavage, side view. M. — Parascaris equorum; 

 Metaphase of 2nd. cleavage (2-celled embryo). N. — Parascaris 

 equorum; "T" embryo (4-celled) with regular chromosome structure 

 in cells P2 and EM, and "diminution" divisions in cells A and IJ. 

 O. — Parascaris equorum; Metaphase of "diminution" division in 

 cell SI. P. — Parascaris equorum; Anaphase of "diminution" 

 division in cell EM. Q. — Parascaris equorum; "Lozenge-Shaped' 

 embryo (4-celled) with only cell P2 not showing "diminution' 

 R. — Parascaris equorum; 3rd. cleavage, cell EM with "diminution" 

 spindle. S. — Parascaris equorum; P2 undivided, the other cells 

 showing chromatin elimination following "diminution" divisions. 



T Parascaris equorum; Blastula in section (only "germ cells" 



not showing evidence of "diminution' division). U. — Rhabdias 

 bit fou is; Prophase of 1st. spermatocyte. V. — Rhabdias bufonis; 



Metaphase plate of 1st. spermatocyte. W. — Rhabdias bufonis; 

 Anaphase of 1st. spermatocyte (heterochromosomes lagging). X. — 

 Rhabdias bufonis; Ilnd. spermatocytes (sister cells). Y. — Rhabdias 

 bufonis; Metaphase of Ilnd. spermatocyte. Z. — Rhabdias bufonis; 

 Anaphase of Ilnd. spermatocyte (heterochromosomes lagging). 

 AA. — Rhabdias bufonis; Telophase of Ilnd. spermatocyte (hetero- 

 chromosomes lagging). BB. — Rhabdias bufonis; Spermatids show- 

 ing cytoplasmic reduction (heterochromosome lost with the lobe in 

 half of the cells). CC. — Rhabdias bufonis; Dimorphic spermatozoon 

 (large one retains the heterochromosome; n = 6). DD.- — Rhabdias 

 bufonis; Dimorphic spermatozoon (small one loses the hetero- 

 chromosome; n = 5). EE. — Rhabdias bufonis; Metaphase plate 

 of last generation oogonium (12 chromosomes). FF. — Rhabdias 

 bufonis; Prophase of 1st. oocyte. GG. — Rhabdias bufonis; Meta- 

 phase plate of 1st. oocyte (G chromosomes). II. — Rhabdias 

 bufonis; Anaphase nucleus of 1st. oocyte. JJ. — Rhabdias bufonis; 

 Prophase of Ilnd. oocyte (1st. polar body). KK. — Rhabdias 

 bufonis; Metaphase of Ilnd. oocyte. LL. — Rhabdias bufonis; Ootid 

 (female pronucleus, and polar bodies 1 and 2). MM. — Rhabdias 

 bufonis; Ovum (male pronucleus with 5, and female pronucleus with 

 6 chromosomes). NN. — Rhabdias bufonis: Ovum (both pronuclei with 

 6 chromosomes). 00. — Rhabdias bufonis; 1st. cleavage spindle (5 

 chromosomes of male, and 6 chromosomes of female 

 origin). PP. — Rhabdias bufonis; Embryonic "germ cell", 

 nucleus with 11 chromosomes (male pronucleus). QQ. — Rhabdias 

 bufonis; Embryonic "germ cell" nucleus with 12 chromosomes 

 (female). U-PP. modified after Schleip, 1911, Arch. Zellf., V. 7; 

 others original. 



208 



