1 1 8 MONO TREMA TA 



deferentia open into the cloaca, not into a distinct urethral passage. 

 The penis, attached to the ventral wall of the cloaca, is perforated 

 by a canal in the greater part of its length, and not merely grooved, 

 as in reptiles and those birds which have such an organ. The 

 canal is open at the base and brought only temporarily in contact 

 with the termination of the vasa deferentia, so as to form a seminal 

 urethra when required ; but it never transmits the urinary secretion. 

 This condition is a distinct advance on that of the Sauropsida in 

 the direction of the more complex development of these parts in 

 most of the other Mammalia. The ureters do not open into the 

 bladder, but behind it into the dorsal wall of the genito-urinary 

 passage. The mammary glands have no distinct nipple, but pour 

 out their secretion through numerous apertures situated in a cup- 

 shaped depression of the abdominal skin, forming a mammary 

 marsupium, especially developed in the females during lactation. 

 It should be mentioned that, according to the observations of Pro- 

 fessor Gegenbaur, the mammary glands of the Monotremes are the 

 simplest found in the entire class. The region of the glands is, 

 indeed, distinguished from the rest of the abdomen merely by its 

 thicker layers of muscles. The glands themselves are closely con- 

 nected with the hair-follicles, and belong to the sudoriparous type, 

 whereas the glands of all other mammals are of sebaceous origin. 



The young are produced from eggs laid by the female parent, 

 which are meroblastic, like those of birds; that is to say only a 

 portion of the yolk segments and forms the embryo, the remainder 

 serving for the nourishment of the latter. 



The above are the principal distinguishing characters of the 

 group, and apply not only to the subclass, but of course equally to 

 the one order Monotremata, in which the two existing genera are 

 included. In addition to these more important characters, the 

 following minor features may also be mentioned. 



The scapula differs from that of all other mammals in that the 

 ridge corresponding to the spine of other forms is situated on the 

 anterior border instead of in the middle of the outer or dorsal surface. 

 The humerus is much expanded at its two extremities, and has a very 

 prominent deltoid crest, and a well-marked entepicondylar foramen. 

 The dorso-thoracic vertebrae are nineteen in number, and have 

 no terminal epiphyses to their bodies. The tranverse processes of 

 the cervical vertebras are of autogenous formation, and remain 

 suturally connected with the remainder of the vertebra until the 

 animal is full-grown. Though in this respect they present an 

 approximation to the Sauropsida (Reptiles and Birds), they differ 

 from these classes, inasmuch as there is not a gradual transition from 

 these autogenous transverse processes of the neck (or cervical ribs, 

 as they may be considered) into the thoracic ribs, for in the seventh 

 vertebra the costal element is much smaller than in the others, 



