‘ss The American Naturalist. ri [February, 
EMBRYOLOGY.! 
Development of Mammals.—In so thoroughly worked over and 
so narrowly bounded a field as vertebrate embryology we should hope to 
find a singleness of plan running through the series, accompanied by 
an agreement amongst workers and theorists as to the interpretation of : 
the known phenomena. In fact, however, the greatest possible 
divergence is found. This is especially marked in the recent attempts 
of embryologists to explain the process of gastrulation in the groups of 
vertebrates. Of course the problem of the mesoblast is here, as every- 
where, a challenge for battle ; but this is not all, for even the origin of 
endoderm and ectoderm have their various interpreters. fee 
An illustration of this is furnished by the three (and more) hypotheses 
which are advanced to account for the early stages of development of — 
the mammals. Two of these may be taken here as an example, and 
a third will be mentioned below in a review of Hubrecht’s recent papet. 
The two which we shall now consider are those of Haddon * and 
` Minot.3 These two theories, advanced about the same time, are said ee 
by Haddon to be ‘‘somewhat similar hypotheses,’’ and Minot says 
that his own is ‘‘ the most satisfactory, and preferable to the similar 
explanation advanced. . . by Haddon.” To an outsider the two 7 k 
theories seem to contradict each other in all that is essential and pE 
to each. 
Balfour prophesied that the ancestral mammal had a large | ovum 
filled with yolk, and this, by Caldwell’s discovery of the eggs of M 
tremes, has been practically demonstrated. Both Haddon and Minot 
accept this as their starting point, but immediately diverge in ee 
directions in their ‘somewhat similar explanations.’ 
The two accompanying diagrams have been cop 
views : ie 
Diagram A gives Haddon’s idea of the meaning of the : 
layers of the mammalian embryo. The central cavity (y-5- 
-sac of the ancestral vertebrate, which has been covered over A 
cociously by ectoderm (e.c.); ancestrally this was accomplished by ee 
bole. At the upper pole the blastoderm, owing to the pepe 
has fallen into the yolk-cavity, leaving a small’ opening (3! n, 
1 Edited by T. H. Morgan, Johns Hopkins University, Baltimore: 
2 Elements of Embryology 
3 AMERICAN Nan aiios April, 1889. 
= 
ied to illustrate t g 
