The American Naturalist 



:>>-: 



Some of the researches in this subject seem to be of such 

 interest, though but beginnings and liable to be wrongly 

 valued one way or the other, that a review of them here may 

 aid in calling attention to a comparatively new line of 

 research, one that is as yet in the limbo of pathology and thus 

 excluded from zoological and embryological text books. 



Passing over numerous experimental investigations upon 

 the hen's egg, some of which appear to have resulted in the 

 formation of definite, predictable monstrosities from localized 

 interference with the embryo, we will mention only the work 

 of Leo Gerlack 1 who finally devised a movable window, 

 embryoscope, that allows the chick to be observed and also 

 experimented upon from time to time while continuing to 

 live, at least for 13 days. 



With the aid of this instrument embryological problems 

 such as the origin of the vascular system from parablast or 

 from the primitive streak may be approached experimentally, 

 by destroying the primitive streak for instance. By similar 

 methods the author hopes to produce changes in embryos of 

 several successive generations and thus strive towards the 

 selection of important questions in heredity. 



It is the frog, however, rather than the chick which has 

 given more decided answers to physiological inquiry promis- 

 ing to be in its early stages what it has become as 

 adult, an easily accessible and not so excessively equivocable 



Professor Pfliiger, 2 starting from the observed facts that 

 frogs' eggs taken from the uterus and thrown into water, float 

 at first with variously inclined axes but after fertilization 

 turn so that the black pole is uppermost and the white pole 

 downward and that when cleavage takes place the first and 

 second planes are vertical, the third horizontal was led to 

 inquire what connection there may be between cleavage planes 

 and gravitation. 



The method of investigation was simply to remove ovarian 

 eggs with their gelatinous capsules and fix them by their own 



Pfliige. 

 • Archw.f.Phjrfc.m, 1883"; 



, 1888, pp. , 



