June 20, 1902.] 



SCIENCE. 



981 



distinct sheet-like germ, which seems to be 

 lacking to the germ grains so far as their 

 development is known. These latter are 

 in the beginning solid, but their first act 

 of independent life seems to be a hoUowing- 

 out by which they become converted into 

 thick-walled vesicles, observed in the case 

 of Cercaria and Bucephalus. The germ 

 of a true egg is also to be looked upon as a 

 vesicle, which in the case of birds only 

 gradually grows around the yolk, being 

 supplemented in the beginning by the 

 vitelline membrane. Since, however, the 

 germ is the undeveloped animal itself, we 

 cannot assert, without good reason, that the 

 simple vesicular form is the common ances- 

 tor from which all animals are descended. 

 The germ grain goes over into this prim- 

 itive condition on account of its own in- 

 herent power; the egg, only after its fe- 

 male nature has been neutralized by fer- 

 tilization. Not until this has occurred does 

 the separation into germ and yolk, or body 

 and nutrient stuff, take place. 



(/) If in order to find resemblance be- 

 tween two animals, we must go back in the 

 history of development a distance propor- 

 tional to the amount of difference 

 they display in their adult condition, we 

 must recognize as laws of individual de- 

 velopment : 



1. That those characteristics common to 

 a large group of animals appear earlier in 

 their development than those which char- 

 acterize the members of the group individ- 

 ually. 



2. That from the general, the less gen- 

 eral is formed, until what is most special 

 appears. 



3. That the embryo of every animal, 

 instead of passing through the adult stages 

 of others lower in the scale, in reality 

 grows increasingly different from these. 



4. That the embi-yo of a higher animal 

 never does resemble the adult of a lower 

 one, but only its embryo. 



It is only because the less highly devel- 

 oped animals go little beyond their embry- 

 onic condition, that they present certain 

 points of similarity with the embryos of 

 higher forms. These resemblances there- 

 fore do not indicate the existence of a 

 limiting condition determining the course 

 of the development of the higher forms, 

 but find their explanation in the organiza- 

 tion of the lower ones. 



(</) These facts are illustrated graphic- 

 ally in a table shoAving the advance from 

 the lowest grade of development to the 

 highest. From this schema it is clear that 

 an embryo cannot be maintained to pass 

 in its development through the whole series 

 of animals, because it cannot pass from one 

 fundamental type over into another. Then 

 again an embryo in its development does 

 not pass through another form but only 

 through the region of indifference be- 

 tween that form and its own adult condi- 

 tion. Thus the further the development 

 proceeds, the narrower does the region of 

 indifference become. The schema also dem- 

 onstrates that an embryo in the beginning 

 is an indifferent vertebrate, then an indif- 

 ferent bird, and so on. Since in its pro- 

 gression from one region of indifference 

 to the next it is becoming internally more 

 and more perfect, it is at the same time 

 also becoming a more and more highly 

 developed animal. 



The view here advocated differs from 

 the one generally held, in that this is based " 

 on an unproved assumption and derives 

 support from the neglect of the important 

 distinction between type of organization 

 and degree of development. 



The embryo is gradually formed by pro- 

 gressive diversification of tissues and of 

 form, and for this reason the younger it 

 is, the more nearly does it resemble 

 slightly developed animals. Different 

 animals vary more or less from the basal 

 type which is nowhere pure but occurs 



