28 



V. LIFE HISTORY OF THE THREADWORM. 



The threadworm is oviparous ; that is to say, the female 

 deposits eggs which are hatched outside the body of the 

 parent. These eggs are minute oval bodies, slightly flattened 

 at one side. The eggshell consists of three layers, except at 

 one point where the middle layer is defective ; and it is said 

 that the embryo makes its escape at this spot 1 . 



Development \vithin the Egg. 



When deposited by the mother, the eggs 

 contain embryos in various stages of develop- 

 ment. In some, only the traces of commencing 

 segmentation of the ovum can be found 2 ; 

 others may have progressed so far as to show 

 already the coiled-up embryo 3 . But in any 

 case, the process of development of the embryo 

 within the egg is rapid, when the latter has 

 been deposited in favourable surroundings, 

 a certain amount of heat and moisture being 

 the necessary conditions 4 . 



Ova of 

 Oxyuris ver- 

 micular is 

 magnified 275 



diameters. 



( After Eich- 



horst. ) Every 



adult female 



contains 10,000 



to 12,000 of 



these eggs. 



1 " L'acide acetique separe le 

 chorion du reste de la coque, sauf 

 en un point large de 7 /u. situe a la 

 face dorsale de 1'oeuf, en arriere du 

 pole cephalique : a ce niveau, la 

 couche moyenne de la coque fait 

 defaut, en sort que les deux couches 

 externe et interne entrent en 

 contact. Cette particularity de 

 structure a une grande importance : 

 sous 1'influence des acides, du sue 

 gastrique ou de la putrefaction, le 

 point en question se detache a la 

 moindre pression, laissant derriere 

 lui en etroit orifice par lequel 

 1'embryon pourra s'echapper." 

 (BLANC'HARD, R., op. cit., p. 712.) 



2 MAYER states that he has 

 observed spermatozoa amongst the 

 eggs : "Auch bei Oxyuris vermicula- 

 ris sah ich einesehr grosse Anzahl von 

 Sammenthieren, deren Lange 1-100 

 Lime ungefahr betrug, in gekriimm- 

 ter Gestalt init zugespitzten Enden, 



zwischen den Eiern zerstrent liegen, 

 als diese bei der Berstung des 

 Thieres aus dessen Leibe heraus- 

 traten." (MAYER, F. J., Beitraeqe, 

 zur Anatomic der Entozoen, Bonn, 

 1841, p. 6.) 



3 The embryos are, as a rule, 

 fully formed within the eggs before 

 the latter are laid : " D'autres, 

 telles que 1' Oxyuris vermicularis 

 . . . ne pondent leurs oeufs que 

 lorsque 1'embryon est deja tout 

 forme ou qu'il s'est mme depouille 

 des envelloppes de 1'oeuf. II ne 

 faut cepandant pas attacher trop 

 d'importance a cette epoque de la 

 ponte, qui n'est peut-etre pas 

 toujours constante chez la meme 

 espece." (CLAPAREDE, E., op. cit., 

 p. 83.) 



4 The minimum degree of heat 

 necessary is, according to LEUCKART, 

 about 32 R. ( = 104 F.), which 

 is slightly above blood-heat : 



