356 



AVES. 



Fig. 185. 



-d 



FowIS 



excitement is found in those species of which 

 the males have a penis. 



Birds differ from all the other oviparous 

 vertebrata in having the canal which completes 

 and carries out the ovum single, and in this 

 respect they manifest an analogy to many 

 mammalia. When, however, the whole of the 

 circumstances from which this condition re- 

 sults come to be investigated, the nature of the 

 part in the two classes will be found to be 

 widely different. 



In the Mammalia the single efferent canal 

 results from a blending together of the vaginas 

 and uteri of the two sides of the body for a 

 greater or less extent along the mesial line; 

 which junction is continued from the external 

 outlet towards the ovaria, but never extends 

 beyond the uteri, the Fallopian tubes always 

 remaining distinct. And in proportion as the 

 generation approximates the oviparous mode, 

 the efferent tubes remain separate for a greater 

 extent. Thus, among the Kodentia, we find 

 the uterus completely divided into two lateral 

 tubes, as in the Rabbit; and in the Marsupiata 

 the division is continued through the whole 

 extent of the true vagina. 



In the true Oviparous classes the oviducts 

 are always double and open separately into the 

 cloaca, and the exception in the class of Birds 

 to this rule is only apparent. 



At an early period of existence the two 

 oviducts exist of equal size, but the left one 

 alone attains that state of development which 

 qualifies it for the exercise of the sexual func- 

 tions. Hitherto no exception has been found 



* This figure, and those numbered 133, 134, 135, 

 136, 138, 151, 153, 163, 182, are copied from the 

 plates of the second edition of Carus's ' Verglei- 

 chenden Zootomie.' 



to this rule, and the uniformity in the condition 

 of the excluded ovum in Birds corresponds with 

 the sameness which prevails in the structure of 

 the organs concerned in its evolution. 



The ovarium is in general single like the 

 oviduct, and developed only on the left side, 

 as in the Rasores. But two ovaria have been 

 observed in many of the Raptores. In the 

 Falcons NITZSCH found the right ovary more 

 developed than the left, and also m some 

 species of Eagle and Owl. In the Sparrow- 

 Hawk the same distinguished anatomist found 

 two ovaries equally well developed. 



In the Common Fowl the ovary first makes 

 its appearance as a membrane beset with small 

 pellucid vesicles adhering to and apparently 

 developed from the coats of the vena cava. 

 The substance of the ovary is invested by a 

 thin and extensible capsula propria, covered 

 by a reflection of peritoneum. The ova are 

 imbedded in a stroma of delicate and yielding 

 cellular substance, and consist each of a mi- 

 nute pellucid vesicle, surrounded by the yolk, 

 which at this period is as clear as the fluid of 

 the vesicle itself, and both are inclosed in a 

 distinct transparent capsule. 



When the ovum has attained the diameter 

 of a line, the vitelline liquid presents a turbid 

 whitish appearance. When it is about the size 

 of a pea the yolk begins to assume a slight 

 straw-coloured tint, and the seat of this colour- 

 ing matter may be observed to be certain glo- 

 bules of oil now superadded to the albuminous 

 and serous fluid. As the oily material prevails, 

 the yolk gradually assumes a more viscid and 

 tenaceous consistency, and a deeper and deeper 

 tint, until it presents the rich orange colour 

 characteristic of the mature ovarian ovum. 



If one of these ova be transversely divided 

 after being hard-boiled, the cut surfaces of the 

 yolk will present three concentric strata of diffe- 

 rent colours ; the external one is of a pale straw 

 colour, the middle one of a deeper yellow, 

 and the internal one is again light-coloured, 

 and surrounds a substance of a whitish colour 

 and more fluid consistency, from which a canal 

 surrounded by a similar substance is continued 

 to the cicatricula. The central substance and 

 continuous canal are composed of albuminous 

 fluid containing white granules, similar to the 

 colliquamentum of the cicatricula. 



The primitive vesicle of the ovum around 

 which the material of the yolk is accumulated, 

 by no means grows with the growth of the 

 ovum ; it is not more than one-half larger in 

 the largest ovarian ovum, than it was when 

 the ovum exhibited its smallest dimensions, 

 and when the vesicle formed its most con- 

 siderable part. Throughout the whole of this 

 period it is, however, the most important part 

 of the ovarian ovum; forming the essential 

 element of the cicatricula, and the centre from 

 which all subsequent development radiates. 



Purkinge, the discoverer of the ' germinative 

 vesicle,' states that it is most easily detected 

 in the ova of the common Fowl, when they 

 have attained the size of from four to six lines. 

 The vesicle is at this period lodged in a mam- 



