ENTOZOA. Ill 



portion is ever likely to be again introduced into the alimentary canal 

 of that species of animal which can afford it an appropriate habitat. 

 The remainder of the germs doubtless serve as food to numerous 

 minute inhabitants of the water ; and the prolific Entozoa may thus 

 serve these little creatures in the same relation as the fruitful Cerealia 

 in the vegetable kingdom stand to higher animals, ministering less to 

 the perpetuation of their own species than to the sustenance of man. 



The nematoid Entozoa present, perhaps, the most favourable sub- 

 jects for studying, with the requisite attention, the successive steps of 

 impregnation, and of the processes by which the germinal vesicle and 

 yolk become finally transmuted into the young and active worm. 



I described and showed diagrams of some of these changes in the 

 ova of the Srongylus inflexus in my lectures on Generation in 1840. 

 The subject has been carefully prosecuted by Professors Siebold * 

 and Kollikerf, from observations made upon the ova of the Stron- 

 gylus auricularis and the Ascaris acuminata, both of them viviparous 

 species of Nematoidea, and subsequently by Dr. Nelson with great 

 care and ability, in the oviparous Ascaris mystax.\ 



The blind end of the tubular ovarium detaches from its inner 

 surface, and contains, minute round cells, which, as they proceed 

 along the tube, enlarge, become nucleated, and surrounded by a 

 finely- granular yolk matter, in which the cell floats as the 'ger- 

 minal vesicle ' with its nucleus. In the more advanced part of the 

 ovarium, these ova are discoid, of an irregular form, and disposed 

 either in simple rows, one behind another, or are grouped round a 

 stem or rachis in the centre of the tube, according to the species. 

 In the oviduct, the ova acquire a colourless chorion, the secretion of 

 that tube. A short diverticulum proceeds from each pole of the 

 ^^^g in Trichosoma and Trichocephalus. 



In the fundus, or beginning of the uterus, much seminal matter is 

 accumulated in the impregnated females. § Dr. Eschricht describes 

 fifty ova as forming a single whorl or wreath in the Ascaris lumhri- 

 coides ; but in the Ascaris mystax Dr. Nelson found but four flat- 

 tened ova in the same plane, filling the transverse section of the 

 ovarian tube. They assume, by mutual pressure, a triangular form 

 with the rounded base next the walls of the tube. In the contracted 

 part of the canal, answering to that which I have called "oviduct" 

 in the Strongylus injiexiis ||, the ova in the A. mystax become 

 separated, and pass in single file to the uterus, altering their form 

 during the passage, in which they first meet the spermatozoa ; but 



* LXXXV. t LXVI. \ XCI. 



§ XXIV. p. 151. II LXXXIV. p. 74. 



