NEMATODA 239 



1. c,, s. 615) found numerous embryonal round worms in the stomach 

 of a cat, 35" in length, and in addition all the intermediate stages 

 of growth up to the larger examples found in the small intestine. 

 They remain in the stomach until they have attained a length 

 of from ' to '' and then pass into the small intestine. When 



18 I * IT 



they have attained a length of nearly \" they cast their skins 

 and change the tooth-like boring apparatus for the three cha- 

 racteristic semicircular lips. These observations on Ascaris 

 mystax (adds Heller) render it probable that A. lumbricoides is 

 also introduced into the human alimentary canal while still in the 

 embryonal state or somewhat further advanced (und wohl auch 

 grosse)." The subject will be found more fully discussed in 

 my account of the large species further on. The cat's worm 

 possesses an historical interest, not only in connection with 

 Bellingham's original discovery, but also in respect of Nelson's 

 subsequent determinations as to the precise mode of impregna- 

 tion in nematodes. The subject is too extended and too special 

 to be dealt with here at any great length. 



For several years after Nelson left the shores of England to 

 spend a too short life in New Zealand, the points discussed in 

 his 'Edinburgh Thesis' (and subsequently published in the 'Phi- 

 losophical Transactions ') formed the subject-matter of numerous 

 memoirs contributed to the leading German scientific journals. 

 Stated with brevity, it may be said that, according to Nelson, 

 the essential act of impregnation occurs when the thimble- 

 shaped spermatozoa of the male penetrate the unimpregnated or 

 ovarian ovum. This, he maintained, could and did take place 

 at any part of the surface of the unfertilised ovum, since the 

 granular mass of which it was composed, though well defined, 

 did not, at this period, possess a limiting or true yolk mem- 

 brane. Professor Allen Thomson, in a series of papers (some 

 contributed in the German language), supported Nelson's views 

 generally. 



Amongst Nelson's chief opponents was Meissner, who demon- 

 strated that the unimpregnated ova really possessed a delicate 

 limiting membrane, and that consequently the action of the 

 spermatozoa was restricted to that portion of the ovarian ovum 

 which became exposed by rupture or separation from the rachis. 

 This opening he termed the micropyle. The union of the 

 sexual elements is quickly followed by a condensation of the 

 yolk-granules, and by the disappearance of the hitherto cen- 

 trally placed germinal vesicle. The ovum next assumes a dis- 



