44 SUMMARY OF CURRENT RESEARCHES RELATING TO 



effected solely by the yolk-sac, as has been recently demonstrated by 

 H. F. Osborn and Caldwell. 



Hypertrophy and its value in Evolution.* — ^Mr. J. B. Sutton, 

 after citing a number of more or less well-known cases of hypertrophy, 

 comes to the conclusions that: — (1) In the lowest form of animal life 

 hermaphroditism is the prevailing condition. (2) Cross-fertilization 

 in hermaj)hrodites is the rule, and may, as with some Myzostomata, 

 lead to division into sexes within the limits of a single group. 

 (3) Sporadic cases of hermaphroditism are far more common in the 

 lowest forms of life. (3) If in mammals both sets of organs grow 

 concurrently, the individual is sterile. (5) Both sets of organs grow 

 equally to a definite period in embryonic life. (6) Eeproduction in 

 vertebrates, so far as is known, is impossible unless hypertrophy of 

 one set of organs occurs. The aim of the author in writing this 

 essay is to try and substantiate the doctrine that pathological processes 

 do not exist per se, but are in all cases to be regarded as physiological 

 processes in excess. Pathology has so far played a part among the 

 ordinary processes of evolution, that hypertrophied organs have been 

 in some cases inherited. 



Availability of Embryologieal Characters in the Classification 

 of the Chordata.f — Mr. J. A. Eyder shows how complication after 

 complication has been added to the developing germ, starting with a 

 simple blastula developed by simple cleavage in the lancelet ; in the 

 amphibian and marsipobranch embryo there is a distinct neurenteric 

 canal, and the neurenteron is continued into the enteric cavity, which 

 traverses longitudinally the upper half of the segmented vitelline 

 mass. In the next grade (Icliihyes) the vitellus is for a long time 

 unsegmented, and is practically excluded from forming any part of 

 the enteric walls ; but the embryo is generally sessile, and while only 

 part of the blastoderm leads to the differentiation of the embryo, no 

 part of the ectoblast is ever folded off to form such provisional organs 

 as the amnion. In the higher (endocyemate) types, where this does 

 obtain, there is ordinarily a blastoderm with a relatively very large 

 area, and only a small part of the ectoblast takes a permanent share 

 in the formation of the embryo. In the Paratherian series (reptiles, 

 birds, ornithodelphs) there is a large yolk developed, which seems to 

 have determined the development of the hollow yolkless blastosphere 

 of the Eutheria; the greater part of the walls of this vesicle are, by a 

 process of folding off and ingrowth of the embryo, converted iuto a 

 respiratory apparatus and secondary system of deciduous envelopes. 



The form of the placenta seems to depend on several factors : — 

 (1) The early or late attachment of the blastodermic vesicle to the 

 uterine walls; (2) the early or late invagination of the embryo; 

 (3) the extent of subzonal membrane covered by the allantois, and 

 the mode in which the latter is extended ; (4) the form of the uterine 

 cavity; (5) the position and disposition of the uterine mucosa; 

 (6) the disposition of its crypts and folds; (7) the arrangement of 



* Proc. Zool. Soc. Lond., 1S85, pp. 432-45. 



t Amer. Natural., xix. (1885) pp. 815-9 and 903-7. 



