296 LECTURE XII. 



MM. Prevost*, Eusconif, and Vogt| have availed themselves, in 

 the investigation of the nature of that mysterious act, and of the 

 changes thence ensuing in the impregnated ovum of fishes. The 

 first change observed when the ovum falls into the water is a separa- 

 tion of the outer covering, or ' chorion,' from the inner tunic or 

 ' membrana vitelli.' It is probable, from observations on other 

 animals, but not proved in respect to the ova of fishes, that the 

 contents of the sperm-cell (body of the spermatozoon) enter the 

 ovum. After the contact of the spermatozoon with the ova, the fol- 

 lowing changes take place in the germ-vesicle. It loses its unim- 

 pregnated character, becomes opaque, enlarges, and, in the ovum of 

 the Tench, according to Rusconi, forms with surrounding granular 

 matter an intumescence beneath the membrana vitelli. Rusconi 

 does not, indeed, interpret this appearance as due to development and 

 metamorphosis of the germ-vesicle and its nuclei ; but Vogt has con- 

 firmed, by observation of the ova of the Corregonus, the important 

 discovery by Barry of the part which the germinal vesicle and its 

 nucleus or nuclei take in the first steps of embryonic development. 

 " The small granules," observes Rusconi, " previously dispersed 

 through the yolk, now become collected at the base of the intu- 

 mescence {fig. 78, a). Half an hour 

 after the occurrence of this first 

 change, two furrows intersecting each 

 other at right angles appeared at the 

 prominent part of the yolk (ib. b, c). 

 A quarter of an hour later, two other 



First steps in the development of a f"i'rows wcre obscrvcd at each side of 

 Tench : Rusconi. ^|^g gj.g^^ g^ ^^^^^ ^j^g prominent part of 



the yolk, which previously was divided 

 into four lobes, now appeai*ed to be formed of eight lobes (ib. d). 

 After the lapse of another quarter of an hour, each of these eight 

 lobes was seen to be subdivided into four smaller lobes, by the for- 

 mation of six fresh furrows, intersecting each other at right angles. 

 At the end of another half-hour, several new furrows appeared, which 

 crossed those which already existed, and subdivided the lobes of the 

 prominent part of the yolk still further, rendering them so small and 

 numerous that it was now scarcely possible to count them (ib. e). 

 The process continued until the surface of this portion of the yolk 

 regained the smoothness which it had before the first furrow ap- 

 peared." 



If this account, abridged by Professor Muller§ from the Memoir 

 of the celebrated Italian physiologist, be compared with the descrip- 

 tion of the first steps in the development of the intestinal worm 



* cxxx. f cxxxi. I cxxxn. § Lxxiv. ii. p. 1510. 



