Class 6. Mammalia. 493 



of the cloaca, the urinogenital sinus, corresponding to the 

 same organ in the female. In connection with the ventral wall 

 of the cloaca there is a penis, wbich differs from the homologous 

 organ of the Ohelonia, Orocodilia, and Aves, in the closure of the 

 groove on the dorsal side, to form a tube, the seminal tube, opening 

 anteriorly into the urinogenital sinus, at the other end to the exterior 

 (Pig. 401, .B). An elongate mass of fibrous connective tissue, the 

 corpus fibrosum, lies ventral to the seminal canal. In all other 

 male Mammalia, including the Marsupials, there is no cloaca; the 

 original opening of the urinogenital sinus into it has closed (Fig. 

 401, 0), and the sinus itself only opens to the exterior by the 

 seminal tube, through which the excretory products must also pass. 

 The copulatory organ is ventral to the anus, but in many Mammalia 

 it is connected with the abdominal wall (X>), so that the tip is directed 

 forwards. 



The seminal tube is surrounded by a vascular network, and one is contained 

 in the corpus fibrosum. The dilation of the vessels effects the erection of the penis. 

 An ossification {os penis) is often developed in the copulatory organ (Camivora, 

 Apes). Various glands open into the urinogenital canal and into the seminal 

 tube, and their secretion escapes with the spermatozoa; among these the 

 prostate glands and Cowper's glands are the most constant. A vesicula 

 seminalis, a saccular or branched hollow organ, which serves both as a 

 seminal reservou-, and also as an organ of secretion, opens, in many Mammals, 

 into each vas deferens, close to its union with the urinogenital sinus ; or inde- 

 pendently into the sinus. A larger or smaller rudiment of the oviduct {uteruB 

 masculinus) is often present. 



Of the Mammalia, the Monotremes alone are oviparous ; 

 here the egg is relatively large,* and segmentation is partial ; as in 

 many Reptiles the egg is surrounded by a parchment-like shell. All 

 other forms are viviparous; an egg-shell is always absent, 

 the ovum is microscopic, segmentation total. In the Marsupials 

 the embryo lies in the uterus surrounded by the embryonic mem- 

 branes, is nourished and grows by the absorption of a fluid secreted 

 by the uterine glands ; there is not a close connection between the 

 embryo and the uterine wall, and the young one is born in a condi- 

 tion which, in comparison with that of the new-born placental 

 Mammal must be considered very undeveloped ; it is nourished by 

 the milk of the parent for a long time after birth. In the 

 placental Mammals the outer embryonic membrane comes 

 into close connection with the uterine wall ; delicate vascular branched 

 villi are developed on its surface and fit into corresponding crypts in 

 the vascular wall of the uterus, serving as organs for the absorption 

 of the maternal plasma. The villi are either uniformly distri- 

 buted over the whole surface, as in the Horse, Pig, Camel, Whale, 

 or are chiefly or exclusively developed in one region, which is then 



* In Echidma the egg with its shell has a longitudinal diameter of 15 m/m., a 

 transverse diameter of 13 m/m, ; that of Ornitjioryncjius is similar. 



