THE EARLY DEVELOPMENT OF THE MAESUPIALIA. 93 



enough to provide room for tlie development of an embryo 

 without any secondary growth in size after it leaves the ovary. 

 Moreover we have to remember that after it has become 

 enclosed in the shell, it remains but a short time in the oviduct 

 and receives little or no additional nutrient material from the 

 oviducal walls. The yolk-mass in any case retains its solid 

 character; there is no necessity for its rapid enclosure, and 

 so enclosure is effected slowly, contemporaneously with the 

 differentiation of the embryo. 



In the Monotreme the conditions are altog-ether different. 

 The ripe ovarian ovum when it enters the oviduct has a 

 diameter of about 3*5 to 4 mm., and is thus considerably 

 smaller than that of a Keptile of the same size as the adult 

 ]\ronotreme. The amount of yolk which it is capable of con- 

 taining is not anything like sufficient to last the embryo 

 throughout the developmental period, and, moreover, it does 

 not provide the space essential for the development of an 

 embryo on the ancestral Eeptilian lines. As Assheton ('98, 

 p. 251) has pointed out, " the difference in size between 

 the fertilised ovum of a reptile or bird or of a mammal 

 is very great ; but the difference in size between the 

 embryo of, say, a bii-d with one pair of mesoblastic 

 somites and of a mammal of the same age is comparatively 

 small. This means that neai'ly the same space is required 

 for the production of the mammalian embryo as of the 

 Sauropsidan, and has to be provided." In the Monotreme 

 not only is additional room necessary, but also additional 

 nutrient material, sufficient with that already present in the 

 egg to last tlie embryo throughout the period of incubation. 

 Both are acquired contemporaneously during the sojourn of 

 the eg*g in the uterine portion of the oviduct, wherein the egg 

 increases greatly in size. When it enters the uterus, the 

 Monotreme egg has a diameter, inclusive of its membranes, of 

 about 4-5 mm. ; when it is laid, it measures in Ornitho- 

 rhynchus, in its greatest diameter, 16-19 mm., and somewhat 

 less in the case of Echidna. Prior to the enclosure of the yolk 

 the increase in diameter, due to the accumulation of fluid in 



