118 MONOTREMATA 
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 vertebree are nineteen in number, and have 
no terminal epiphyses to their bodies. The tranverse processes of 
the cervical vertebre 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, 
