NEMATODA. 683 



intestine grows to a length of one inch. A similar prolapsus appears to 

 occur in the female Simondsia from the stomach of the Pig. The male 

 Trichodes crassicauda passes bodily into the uterus, and the male Syngamus 

 trachealis has been said, but probably erroneously, to undergo fusion with 

 the female 1 . 



The ova originate in the upper part of the ovary, and it is said from a 

 syncytium disposed like the corresponding part in the male. In the small 

 Nematoda there is a single row of ripening ova connected by a slender 

 rhachis ; in the larger a number which are grouped round a central simple 

 rhachis. The ova gradually increase in size, and their protoplasm becomes 

 more or less granular as they approach the oviduct, where they are set free. 

 A vitelline membrane is sometimes formed before impregnation, sometimes 

 after. There is in the latter case a micropyle, which E. van Beneden 

 states is closed by the fusion to its margins of the membrane covering the 

 conoid process (ante, p. 68 1) of the entering spermatozoon. There is an egg- 

 shell which is thin where the embryo hatches in utero, and in some other 

 instances. In other cases it is thick, and then often ornamented externally, 

 and brownish in colour, and lined by one or two internal membranes 2 . 

 The egg is usually oval in shape. The number formed varies, and the size of 

 the worm does not necessarily regulate the size of the ovum. Segmenta- 

 tion may not begin until some time after the laying of the egg, e. g. in 

 Ascaris lumbricoides ; it takes place before laying, as in Dochmius or the 

 embryo may be already formed as in Oxyuris, or even escape from the shell 

 in utero y e.g. Ollulanus, Pseudalius^ Trichina^ Dracunculus, Filaria Ban- 

 crofti. Segmentation is total, and the odd numbers are found among the 

 blastomeres. The latter possess the power of amoeboid motion. In some 

 cases where the embryoes undergo development in utero, the rest of the 

 maternal viscera are destroyed by their growth and movements, but the 

 cuticle persists, and is turned into a moving sac of embryoes, e. g. in the 

 female form of Angiostomum*. 



1 The body of the female Sphaerularia was supposed by Sir J. Lubbock to be an adherent 

 male. As to Syngamus, it is difficult to see how the ova escape, as they undoubtedly do escape, if 

 fusion occurs. 



2 How the egg-shell and its contained membranes arise is a matter of dispute. According to 

 E. van Beneden, the egg itself of A. megalocephala gives origin to a delicate vitelline membrane, and 

 then successively to two peri-vitellar envelopes, its protoplasm retreating after each formation, and 

 leaving a space filled by liquid. The shell proper is formed by the uterus. The same view is held 

 by Leuckart. From Schneider's account, it seems that the thin egg-shell is a vitelline membrane ; 

 the thick, a vitelline membrane modified by subsequent growth, due to the fact that the proto- 

 plasm does not retreat from it ; the two internal membranes are formed at a later period. If, he says, 

 the impregnated egg of A. megalocephala is removed from the uterus and suffered to lie in water, 

 the thick shell developes as usual, thus proving its non-uterine origin. 



8 The history of the embryonic development presents difficulties. Nusbaum speaks of an 

 invaginate Gastrula in Ascaris megalocephala (Z. A. vi. 1883). Hallez observed an invagination in 

 the same animal, and the derivation of both oesophagus and rectum from endoderm (C. R. 101. 

 1885). In Cucullanus elegans, Butschli states that there is a plate formed of two layers of cells, epi- 



