I 



CHAP. XXI.] EMBRYOLOGY, 253 



highly developed in which differentiation, that is to say 



the distinctness of the different organs and tissues, has 



been carried furthest ; and those species are the lowest, or 



least developed, in which the original homogeneous germ 



has undergone the least differentiation, and remains most 



nearly in the original state. Consequently the undeveloped Embryos 



embryos of the higher forms bear some degree of resem- f^^^f ^^ 



blance to the mature states of the lower ones. That is to resemble 



low6r 



say, an organism which has just begun a course of what is forms, 

 destined to be very high development, has some resemblance 

 to one which has completed a much lower course of de- 

 velopment. This is a very general law, and it is, in a 

 great variety of cases, carried out into a degree of detail 

 that we could not have expected to find. 



The germs of those organisms which have any decided Develop- 

 structure do not at once begin to develop into the structure j^^^^ect j^ 

 which they are ultimately to attain. The germ at first most cases. 

 acquires a simple cellular structure, and grows by the 

 multiplication of its cells.^ In this condition it resembles 

 the mature states of the Protozoa and Protophyta (the 

 simplest animals and plants, without any distinction of 

 tissues except that of inside and outside of cell) ; and this 

 cellular mass is afterwards transformed into the organs of 

 the developing organism. Thus the process of develop- 

 ment is seldom or never perfectly direct ; that is to say, 

 the germ does not at once begin to transform itself into the 

 organism that is to be. In some groups it is much more 

 direct than in others ; and, on the whole, the lower the Generally 

 organization the more nearly direct is the development : ™eTrlv 

 but this rule is subject to so many modifications and ex- direct in 

 ceptions (due, as I believe, to causes that I shall endeavour groups, 

 to set forth at the end of this chapter), that it cannot be 

 stated as even an average or general truth. Among the Process 

 lower aquatic Invertebrata generally, the germinal mass ji^^r^l^. ^ 

 early loses its ceUular structure (if indeed it ever possessed vertebrata. 



1 Carpenter's Human Pbysiology, p. 4, and Comparative Physiology, 

 p. 176. All vegetable organisms, and the higher animal ones, appear to 

 pass throngh this cellular stage of development, but it is certainly not 

 proved of the lowest animals. 



