tures and humidities. The hydrophilic pi'operties of 

 the protein allow the retention of moisture, and it is 

 only after the membrane has been dried that desicca- 

 tion of the embryo begins. 



Figure 1.3C. 



A — The penetration of the sperm into the highly vacuolate 

 vitellus of the egg. B — The tetrad of the first polar body. The 

 shell and the vitelline membrane have completely formed. The 

 vitellus has condensed toward the center of the egg leaving a 

 space, the peri-vitellus space, between the vitellus and the 

 vitelline membrane which is diagrammatically shown. The chromatin 

 of the male nucleus is condensed and surrounded by granular 

 archoplasm. C and D — ^The division of tetrads to form the first 

 polar body. E — Dyads after the first polar body is given off. These 

 divide to form the tetrads of the second polar body. F — The male 

 and female pronuclei and the two polar bodies, the first against 

 the vitelline membrane and the second in the vitellus itself. 

 Fertilization (Fig. A) occurs in the oviduct. All subsequent stages 

 are found in utero. Original, Christenson. 



Abbreviations : inl, protein layer ; sh, shell : vit in^ vitelline 

 membrane ; op, operculum. 

 Original, Christenson. 



Oviparity 



R. O. C. 



By far the majority of parasitic nematodes are ovi- 

 parous. In some (Trichuris trichmra, AsHaris lumbri- 

 coides, et al.) the eggs are discharged in the unsegmented 

 condition. After variable periods of incubation outside 

 the host, usually of long duration, infective embryos are 

 produced. Other species (Necator americaniis, et al.) 

 t.evelop rapidly, hatching occurring in a iew days. The 

 liberated larfae undergo further development in the soil. 

 In some spe;ies partial intra-uterine development of the 

 larvae occurs. Typical morulae, for example, are pro- 

 duced in the eggs of Contracaenan quadricuspe, and 

 llabroncma colaptes before they are discharged (Walton, 

 1923). In utero development may progress until the eggs 

 contain infective larvae in some forms. Such is the 

 case with Enterobius vermicularis and many other spe- 

 cies. Intermediate hosts are involved in the transfer of 

 many nematodes, either the embryonated eg-gs or the 

 infective larvae being taken up by annelid worms, arthro- 

 pods and other essential hosts. 



The condition of oviparity approximates ovoviviparity 

 in some parasitic nematodes. Development progresses 



until larvae are produced and these hatch almost imme- 

 diately after oviposition. This condition is seen in 

 Rhabdias bufonis, a pulmonary parasite of Amphibia, or 

 in Rhabditella axei (syn. Rhabditis macrocerca) as re- 

 ported by Faust and Martinez (1933). After the dis- 

 charge of the eggs in the lungs, in the case of the 

 former species, they traverse the alimentary canal usu- 

 ally hatching in the rectal portion. Intra-host hatching 

 likewise occurs in the case of Strongyloides stercoralis 

 and Spirocerca Ixpi (S. sanguinolenta) . Following gastric 

 infections with the latter species the embryos are found 

 free of the egg membranes in the caecum and colon of 

 dogs. This parasite is likewise found commonly in aortic 

 nodules from which the embryonated eggs are discharged 

 directly into the blood stream where they hatch. The 

 mi;rofilaria-like, unsheathed larvae have often been con- 

 fused with those of filarioid worms (Lewis, 1874, et al). 



Ovoviviparity 



R. O. C. 



Ovoviviparity refers to the intra-uterine hatching of 

 fully formed eggs. It is a wide-spread phenomenon among 

 nematodes parasitic in tissues. Miany of the so-called 

 viviparous species are, in the strict sense of the term, 

 actually ovoviviparous. 



A good example of ovoviviparity is afforded by Cosmo- 

 cercella haberi. In this species Steiner (1924) reports 

 from one to three larvae, and one to five eggs in the 

 uterus at one time. The case of Trichinella spiralis is 

 complicated by a greater biotic potential but is none the 

 less clear. Immature females possess typical eggs in the 

 uterus, each composed of the segmenting protoplasmic 

 mass, a vitelline membrane and a shell (Fig. 141 AA). 

 Dirofilaria immitis and Dracunculus niedinensis are like- 

 wise commonly considered viviparous species which are 

 actually ovoviviparous. In both cases typical eggs form 

 which hatch in utero (Figs. 141 T & N). 



Some of the other well-known, so-called viviparous 

 forms should be included here. The eggs of Onchocerca 

 fasciata are described by Badanine (1938), as covered by 

 an excessively delicate membrane which may present 

 either a spherical or oval outline. No attempt was 

 made to determine the egg membranes present, but the 

 figure given is fairly typical of the eggs of ovoviviparous 

 species. Blacklock (1926, 1939) describes the eggs of 

 Onchocerca volvulus stating that the egg membrane 

 remained practically unstained when dried, fixed in al- 

 cohol and stained with hot haemalum. This procedure 

 would destroy the delicate vitelline membrane leaving 

 the shell which is undoubtedly the membrane Blacklock 

 observed about the coiled embryos. 



Augustine (1937) made some interesting observations 

 of the early development stages of Vagrifilaria cohimbi- 

 gallinae. He observed that the developing larvae in the 

 uterus are enclosed in a delicate membrane but that no 

 membranes were present about the larvae in the vaginal 

 region. He found crumpled, hyaline objects in the vagi- 

 nal part of the uterus which were similar in size and 

 shape to the egg membranes indicating intra-uterine 

 hatching. 



Some authors go so far as to include under ovoviviparity 

 species in which the embryonated eggs hatch outside the 

 uterus. Kreis and Faust (1933), for example, consider 

 Rhabditella axei (syn. Rhabditis macrocera), as being 

 ovoviviparous but state that hatching occurs after the 

 eggs have left the parent worm. 



Whether or not viviparity in the true sense of the 

 ■ term actually exists in the Nematoda will have to be 

 determined through critical research on early develop- 

 mental stages of additional species. 



Significance of the Embryonic Sheath 

 R. o. c. 



Nematodes, like arthropods, grow through the process- 

 of ecdysis. Usually the cuticle is shed four times, the 

 fifth stage thus formed being the adult. The so-called 

 "infective stage" reached before the invasion of the 

 definitive host in many species follows the second molt. 

 This stage generally occurs in the soil {Necator ameri- 

 canus, et al.) , or in an intermediate host. Molting usually 

 follows shortly after the escape of the larvae from the 

 egg envelope but it may occur in the egg itself. This 



179 



