that there is no specially madified structure for shell 

 formation, and that it seemed probable that the chitinous 

 shell was formed from a secretion of the egg itself. 



Faure-Fremiet (1913) reports on the endogenous de- 

 velopment of the vitelline memlcrane of Parascaris equor- 

 i!»t. He states that this membrane is formed from a 

 particular fat body vi'hich pre-exiits in the oocyte under 

 the form of refringent bodies or crystalloids. He names 

 the pre-existent substance extracted from whole gonads 

 ascarylic acid. 



Wharton (1915) gives the best description of the 

 eggs of Ascaris lianbricoides. He describes the eggs as 

 consisting- of a central mass of protoplasm and yolk wdth 

 a very thin vitelline 'membrane, surrounded by a thick, 

 transparent shell consisting of an inner layer of chitin 

 and an outer layer of some alhuminous material. Like 

 Blanchard (1889) he considered the chitinous shell to be 

 composed of two parts, a thin, tough very refractive 

 layer, and a thicker, moi-e brittle outer layer which 

 often showed delicate striatiins. The egg does not 

 completely fill the shell, 'but forms a round ball in the 

 center with a clear space at each end. The polar bodies 

 were observed in this clear space. The pigmentation of 

 the albuminous coat is considersd to be due to the ab- 

 sorption of bile, a view held by Blanchard, since in 

 females kept alive in Kronecker's solution the albuminous 

 coat was colorless at the time of oviposition. 



Thomas (1924) finds three principal membranes com- 

 posing the envelope of eggs of Trichosomoides crassi- 

 cauda; (1) the outer shell which stains black with Heiden- 

 hain's hematoxylin (protein coat), (2) an almost blue- 

 gray fertilization membrane (chitinous shell), and (3) 

 a Ibrownish vitelline membrane. Between the vitelline 

 membrane and the so-called fertilization membrane a 

 peri-vitelline space is described. 



Chitwood (1930), like Nelson (Loc. Cit.), considers 

 that the eggs receive their shells while passing through 

 the oviduct. He does not commit himself as to whether 

 the development is exogenous or endogenous. In un- 

 published manuscript Chitwood points out that shell 

 formation varies; in some species it occurs in the oviduct, 

 whereas in others it occurs in the uterus. The produc- 

 tion of the cutieular shell, he says, can be observed in 

 the uterus of Rhahditis terricola if a mature female is 

 isolated and ohserved for a few hours. The shell first 

 appears as a delicate line which thickens as development 

 progresses. 



Wottge (1937) offers the most critical studies on the 

 development of the eggs of Parascaris equorum. He 

 found that following the penetration of the sperm a 

 clear, transparent shell develops very quickly. This he 

 called the homogeneous membrane. After its appearance 

 the egg does not increase in size. The egg-cell itself 

 shrinks in size and there is a corresponding reduction 

 in the diameter of the homogeneous membrane. The 

 first polar body is thrown off when this layer is com- 

 pleted. A second membrane then appears Taetween the 

 egg-cell and the homogeneous layer which is termed the 

 striated membrane. It also develops rapidly, and is thick, 

 with striations running parallel to its surface. After 

 the production of this layer a wide space, the "Saftraum" 

 or fluid cavity, develops around the egg-cell. When this 

 is established the second polar body is discharged. These 

 membranes remain unchanged during the fui'ther devel- 

 opment of the egg. 



From the above survey it is clear that the vitelline 

 membrane is a zone of condensed protoplasm about the 

 egg-cell (vitellus of Nelson) and that the chitinous shell 

 forms rapidly about it. The diameter of the egg does 

 not increase during shell production as it would if an 

 exogenous process were involved. This was noted by 

 both Nelson (18.52) and Wottge. Both workers describe 

 the peripheral protoplasm during shell formation as 

 vacuolate and granular. A shrinking of the egg proto- 

 plasm away from the vitelline membrane leaves the 

 peri-vitellus space or "Saftraum" of Wottge. The pro- 

 duction of the .=hell in the As'iaridoidea is in the 

 oviduct; in some other forms it may oecur in the uterus. 

 The protein coat forms by the adherence of the uterine 

 secretion to the surface of the true chitinous shell. The 

 brown pigmentation of ascarid eggs, and possibly others, 

 is due to staining by the bile pigments. The so-called 

 fertilization membrane of Thomas is undoubtedly the 

 chitinous shell. The following section gives the sequence 

 of development of the egg membranes. 



LAYERS OF THE EGG MEMBRANE 

 R. O. C. 



The pi-esent conception of the egg envelope is similar 

 to the interpretation of Blanchard. Three coverings 

 should be recognized, namely: (1) the protein external 

 coat which is secreted by the uterine wall, (2) the chitin- 

 ous membrane or true shell which is a secretory product of 

 the egg itself, and (3) the vitelline membrane which is 

 formed in the oviduct and binds the elements of the 

 initial ovum together. In some groups, such as the 

 Rhabditoidea, Tylenchoidea and Strongyloidea, the pro- 

 tein coat is absent in most of the species. It is also 

 absent in ovoviviparous species such as Trichinella 

 ■spiralis, Dirofilaria immitis and Dracunculus medinensis. 

 Jn these forms a true egg of the strongyloid type is 

 produced and hatching occurs in utero. All three mem- 

 branes have been demonstrated in the Mermithoidea, 

 Ascaridoidea, Oxyuroidea, Dioctophymatoidea, Spiruroid- 

 ea, the oviparous Filaroidea, and some of the Strong- 

 yloidea. They are probably also present in some repre- 

 sentatives of the Rhabditoidea, Plectoidea and Tripyloidea. 



The protein external coat is variously modified in 

 different forms. The terminal byssi described for the 

 Mermithoidea (Figs. 139 & 140) are products of this 

 layer and are continuous with the polar thickenings of 

 it (Christie, 1937). Foster (1914) considers that the 

 terminal filaments of the eggs of Tetrameres sp. are 

 not simple prolongations of the chitinous shell but are 

 added after the shell is complete. Our studies of the 

 eggs of CiteUina mannotae and Tetrameres sp. give the 

 same impression. The opercula of the eggs of the 

 Trichuroidea are protein akin Tjut not identical chemically 

 to the protein coat (Chitwood, 1938) and like the latter, 

 the opercula might also be products of the uterine secre- 

 tion. 



The true shell presents little variation in morphology 

 although it may be stratified in appearance. Biochemically 

 the strata of the shell are the same notwithstanding the 

 fact that Zawadowsky (1928) ef al, divide the shell into 

 three layers. The shell forms the general contours of 

 the egg. In the Trichuroidea and Dioctophymatoidea it 

 is discontinuous at the ends and projects collar-like 

 parallel to the long axis forming the opercular aperture. 

 Fig. 141Z shows a longitudinal section through the egg 

 of Capillaria aerophila presenting the membrane rela- 

 tionships to the opercula. The shell is apparently formed 

 of chitin (Chitwood, 1938) or a closely allied substance. 



The vitelline membrane varies in its morphology in' the 

 different types. In some forms (Toxascaris leonina, 

 Toxncara vidpis and Parascaris equorum) it is thick 

 and filled with reticulations in the mature ova. In 

 developing ova of Parascaris equorum the reticulations 

 are not so apparent, but the peripheral zone is filled 

 with granules similar to the so-called "secretory granules" 

 of gland cells. The reticulations are apparently lipoidal 

 since they, with other elements of the vitelline membrane, 

 are dissolved in the ordinary fat solvents. Reticulation 

 to a marked degree can be seen in the eggs of Toxascaris. 

 Similar reticulations, though to a lesser degree, are 

 present in the eggs of some of the Trichuroidea. In 

 some species of parasitic roundworms the vitelline mem- 

 brane appears as simply a wide zone of condensed proto- 

 plasm {Enterobius vermicularis, et al.), which is fairly 

 resistant to mechanical abrasives. In others (Necator 

 americunus, et al) it is a very delicate membrane which 

 is scarcely discernible microscopically. 



The sequence of development is as follows: The egg 

 cells undergo a period of multiplication in the caecal or 

 distal portion of the ovary. As they pass down the 

 ovarian tube along the sides of the rachis each egg 

 accumulates yolk material forming a vitellus. The vitellus 

 is pyramidrl in shape immediately prior to its diseharge 

 into the oviduct and contains a germinal condensation 

 and a nucleus. 



Upon reaching the oviduct the egg is fertilized and 

 the vitellus begins to assume the subglobular shape typi- 

 cal of the species. This change of shane may be due, 

 in part, to the contraction of the vitellus to form a 

 sphere or it may result from the pressui-e exerted fol- 

 lowing the imbibition of vjater (Nelson, 18.52). The 

 shell begins to form immediately after the penetration 

 of the vitellus by the sperm, and results from an endo- 

 genous process. This is evidenced by the appearance of 



175 



