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 the 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 nuchce, 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 1 . 



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 egg, which in its chief 

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

 must not be entirely silent when reviewing their general characters. 

 The ovarian egg 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 egg has fallen into 

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

 of an egg just laid. The white of egg, which is disposed around 

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



1 Of the male organs of propagation in birds those of the cock especially have been 

 often figured; see, ex. gr. DE GRAAF, 1. 1. p. 231, Tab. xvn. ; CABUS Vergl. Zootom. 

 Tab. XVI. fig. 1 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 CABUS Tab. anat. comp. 

 illustr. v. Tab. 7, fig. 2. Compare also on this subject G. G. TANNENBERG Diss. inaug. 

 sistens spicilegium observationum circa paries genitales mazculas Avium, cum tabulis 

 sen., Gottingse, 1789, 4to, and J. MUELLER, in his Memoir already quoted, Ueber zwei 

 versckiedene Typen, &c. 



