350 CLASS XVI. 



groove which then faces outwards. A yellow elastic ligament 

 retracts this tube inwards. The part of the penis which is not 

 everted, consists of two fibrous bodies with a groove that is sur- 

 rounded by spongy tissue. Quite peculiar is the typus observed in 

 the penis of the ostrich of the old world. This is large, conical, 

 without a membranous evertible part; besides tlie two fibrous 

 bodies there is here an elastic body which occupies the space or 

 groove between those parts on the under surface, of which the 

 tissue corresponds with that of the ligamentum nudice, and which 

 forms also the extremity of the penis resembling a glans. The 

 groove on the upper surface of the penis is covered by a spongy 

 venous tissue. When at rest this penis is bent and concealed 

 in a sac of the cloaca^. 



Here we may remark that in birds the external sexual dif- 

 ference is commonly much greater than in the rest of the vertebrate 

 animals, a difference which is especially observable in the marking 

 and colour of the feathers; usually the males are much more richly 

 ornamented. This is most striking when the birds have attained 

 their full size and the capacity to propagate, and is particularly 

 observable at pairing time (in their wedding-dress). 



On the development of birds in the &g^, which in its chief 

 features corresponds with that of the Reptilia Jiaplopnoa (p. 225), we 

 must not be entirely silent when reviewing their general characters. 

 The ovarian ^^g consists, like that of other animals, of the yolk 

 and the germinal vesicle with the germinal spot (see above, pp. 

 4, 5). Whilst still in the ovary the yolk attains its full size; 

 it has been observed that its weight, when the ^gg has fallen into 

 the funnel of the oviduct, is exactly tlie same as that of the yolk 

 of an Qgg just laid. The white of (tgg, which is disposed around 

 the yolk in the oviduct, consists of two layers ; the external layer is 



^ Of the male organs of i:)ropagation in birds those of the cock especially have been 

 often figured; see, ex. gr. De Graaf, 1. 1. p. 231, Tab. xvii. ; Carus Vergl. Zootom. 

 Tab. XVI. fig. J 6, Catalogue of the Physiol. Series in the Museum of the College of Sur- 

 geons in London, iv. PI. 50, fig. i ;— of Mergus merganser in Carus Tab. anat. comp. 

 illustr. V. Tab. 7, fig. 2. Compare also on this subject G. G. Tannenberg Diss, inaug. 

 sistens spicilegium observationim circa partes genitales masculas Avium, cum tabulis 

 aen., Gottingse, 1789, 4to, and J. Mueller, in his Memoir already quoted, Ueher zwn 

 rerschiedene Ti/pen, &c. 



