was observed by Seurat (1914) in Nematodirus mauretan- 

 icvs. Cobb, Steiner and Christie (1923) show this to be 

 the case in the development of Agamermis decaudata. It 

 has also been reported for Ascaris luttihricoides v. siium 

 by Alicata (1935), and in Rhabdias fuscovenosa var. 

 catanensis by Chu (1936), and in other species. Some 

 nematodes molt twice in the egigr, two cuticles being 

 present when the larvae hatch. In some instances the 

 embryonic sheath is retained by the larvae as a loosely- 

 fitting coat. This is especially true of the third stage, 

 infective larvae of certain groups of the superfamily 

 Strongyloidea, the second cuticle being the one re- 

 tained. It is apparently protective in function. Occa- 

 sionally the last two cuticles may be found covering the 

 adult stage preparatory to the last molt. Goodey (1930) 

 reports this to be true for Tylenchinema oscinellae, as 

 shown in figure 135G. 



In the Filarioidea the adult parasites live in locations 

 often not associated with the natural openings of the 

 body. Living young are discharged into the humoral 

 elements. These may be sheathed (Foleyella spp., Setaria 

 spp., Isociella spp., Wuchereria bancrofti, T'hamugadia 

 hyalina, Saurositiis agamae, Loa loa, et al) or unsheathed 

 (Onchocerca spp., Dirofilaria spp., Dipetalonema spp., 

 et al) . These larvae may live in the circulatory system 

 for long periods of time without significant morphological 

 changes, for example Underwood and Harwood (1939) 

 transfused larvae of Dirofilaria immitis into uninfected 

 dogs and found that they would survive over two years. 

 Even unsheathed microfilariae can leave the blood stream 

 and migrate through the tissues as has been demonstrated 

 by Harwood (1932) in the case of Litosomoides sigmo- 

 do7itis. 



Shortly after being taken up by the alternate host the 

 sheath of unsheathed forms, such as Wuchei-eria ban- 

 crofti, is shed. Manson (1884) describes the process in 

 detail as found in Wuchereria bancrofti, stating that it 

 usually occurs within an hour in the mosquito vector. The 

 shedding of the initial sheath in this species is followed 

 'ay a striation of the larval integument. Yamada (1927) 

 and Feng (1936) show that two additional molts follow 



Figure 137. 



Cleavage and early embryonic development of Ascuris lumhri- 

 coides. Figures A, B and C represent the 1-cell, 2-cell and 4-celI 

 stages respectively. Figure D shows a late morula or early blastula. 

 Figure E shows the cleavage cavity in a late blastula which is 

 followed by gastrtllation (Fig. F). Figures G and H give two 

 stages in larval formation, the latter being the infective larva. 

 Original, Christenson. 



the initial shedding of the sheath. In unsheathed forms, 

 for example Dirofilaria immitis and Dracunculus medin- 

 ensis, only two molts occur in the vectors before the 

 microfilariae are ready for their transfer. The first 

 molt involving the initial sheath of Mf. bancrofti, fol- 

 lowed by two additional, has led some workers to doubt 

 that the first "cuticle" is homologous to the other two. 



Penel (1904) advanced the idea that the initial sheath 

 of Loa loa was derived from the vitelline membrane. He 

 removed developmental stages from various levels of the 

 uterus noting that the membrane enlarged with the 

 growth of the larvae. In the vaginal portion of the 

 uterus he recovered the sheathed microfilariae. His 

 conclusion was that the larval investment was derived 

 from the egg membrane. Penel's idea has been carried 

 over to account for the origin of the sheath of Mf. 

 bancrofti (See Fiilleborn, 1928) and other species. 



The modification of the egg membrane accompanying 

 the development of the embryo has been seen in other 

 species. Ransom (1904) shows that as the eggs of 

 Habronema vvuscae develop the egg membrane follows 

 the larval contours producing a sheath-like covering (Fig. 

 138). Faust (1928) notes a similar modification in 

 Thelazia callipaeda. In the case of this species, however, 

 there is a peculiar ballooning at the end, a large vesicle 

 being formed. Faust states that the covering is retained 

 for some time and that it is protective in function. 

 Neither of these authors refer to a possible relationship 

 between the modified egg membrane and the initial 

 larval sheath. They, similarly, do not make a state- 

 ment as to which of the egg membranes is involved. 



Penel's idea that it is the vitelline membrane which 

 forms the microfilarial sheath is not tenable. He, like 

 most workers of his period, did not attempt a critical 

 study of the membranes. Our studies show that the 

 developing larvae of such so-called viviparous species 

 as Dirofilaria imm.itis (Fig. 141T) and Dracunculus 

 medinensis (Fig. 141N) are covered by a true shell 

 within which there is a delicate vitelline membrane. This 

 membrane is discernible only by careful study with oil 

 immersion lenses in formalin-preserved materials. It 

 is visible only at points of separation from the chitinous 

 shell. Furthermore, Blacklock (1939) noted the presence 

 of the egg membrane of Onchocerca volvulus following 

 fixation in alcohol and staining with hot hemalum. It 

 is safe to assume that the external membrane in this 

 species, also, is the chitinous shell since the procedure 

 used would destroy the delicate vitelline membrane. 

 Augustine (1937) observed the shedding of the egg 

 membranes in the uterus of Vcugrifilaria columbigallinae, 

 remnants of the shell being found in the vaginal portion. 



It is thus clear that the chitinous shell is present in 

 the eggs of ovoviviparous Filarioidea and Dracunculoidea 

 which have been studied critically. The pi-esence of a 

 chitinous shell implies the presence of a vitelline mem- 

 brane since the two appear almost simultaneously in the 

 developing egg, and the early vitelline membrane must 

 be considered to be simply the zone of shell formation 

 (p. 175). That the chitinous shell has some elasticity 

 and can adjust to the contours of the developing lai-va 

 has been demonstrated by Penel, Ransom and Faust. 

 This does not necessarily mean that the chitinous shell 

 forms the initial microfilarial sheath. Biochemically the 

 chitinous shell is not easy to distinguish from the 

 embryonic cuticle (Chitwood, 1938) since both are highly 

 insoluble. The critical test, solubility in hot alkalis, is 

 often uncertain with such small materials. 



Some workers express the view that the sheath of 

 Mf. bancrofti is derived from the shed cuticle as is true 

 of subsequent stages. As early as 1874, Lewis noted the 

 presence of the sheath in Mf. bancrofti and its absence 

 in Mf. immitis. He advanced two possibilities of forma- 

 tion, either it was derived as the shed cuticle, or it was 

 derived from the egg envelope. More recently Augustine 

 (1937) points out that in the fresh blood the sheath of 

 Mf. bancrofti is extremely difficult to see, but that as 

 heparinized blood dried the "formation" of the sheath 

 could be followed. The efforts of the microfilariae to 

 push on and back out caused a stretching of the once 

 close-ifitting, inconspicuous outer covering. He further 

 states that it is quite possible that some stretching may 

 occur in the circulation when the microfilariae are tem- 

 porarily trapped in the capillaries. Augustine concludes 

 that the sheath of Mf. bancrofti is comparable to that of 

 infective hookworm larvae — namely, the result of an 

 incomplete ecdy&is. He advances the possibility that the 



180 



