EARLY ONTOGENETIC PHENOMENA IN MAMMALS. | 101 
Between the extremes such as we find them on one hand in 
the Ungulates, where the young blastocyst undergoes an 
immense increase in size before the processes described in 
Chapter III are inaugurated, and on the other hand in certain 
Primates, Insectivora, and Rodentia, where the blastocyst is 
yet uncommonly small when these processes are started, 
every possible gradation has already been observed in differ- 
ent orders of Monodelphia. It may in general be said that 
in the first-named case, when there is an enormous initial 
increase in surface, the changes which the trophoblast under- 
goes and the proliferations to which some of its cells are 
subject are much less considerable, whereas in the second 
ease those changes and proliferations are ever so much more 
intense. 
I have no hesitation in saying that the new functions to 
which the trophoblast must have become adapted, simulta- 
neously with the gradual development of the amphibious 
Protetrapod ancestors into monodelphian mammals, were— 
each in its special significance—of the utmost importance for 
the different lines along which this development could pro- 
ceed. he highest degree of development has been reached by 
those descendants whose trophoblast exhibits a maximum of 
activity, and in whom we at the same time find a maximum of 
useful adaptations in the blastocyst, by which the latter can 
have the full profit of the advantages thus offered by the 
proliferating trophoblast. ‘This combination is not always 
present. Thus we find among primitive monodelphia the 
hedgehog and Gymnura with a very intense proliferation of 
the trophoblast (Figs. 36—38), but deficient in the way in 
which the blastocyst responds to and utilises the facilities 
thus offered. On the contrary, in man and the Anthropo- 
morphe the trophoblastic preparations resemble very closely 
to what the hedgehog shows us, but here the development of 
the blastocyst itself has followed a totally different line, and 
has reached a degree of early completeness and differentiation 
(Figs. 39, 40, 141, 143, 144), which secures to the developing 
embryo, during pregnancy, a combination of the most favour- 
