34 THE ANATOMY OF INVERTEBRATED ANIMALS. 



whole, and consequently constitutes a germ whence the whole 

 body can be reproduced. In more advanced organisms each of 

 the multitude of cells into which the embryo cell is converted 

 at first, probably retains all, or nearly all, the physiological 

 capabilities of the whole, and is capable of serving as a re- 

 productive germ ; but, as division goes on, and many of the 

 cells which result from division acquire special morphological 

 and phj'siological properties, it seems not improbable that they, 

 in proportion, lose their more general characters. In propor- 

 tion, for example, as the tendency of a giv^en cell to become a 

 muscle-cell or a cartilage-cell is more marked and definite, it 

 is readily conceivable that its primitive capacity to reproduce 

 the whole organism should be reduced, though it might not be 

 altogether abolished. If this view is well based, the power of 

 reproducing the whole organism would be limited to those 

 cells which had acquired no special tendencies, and conse- 

 quently had retained all the powers of the primitive cell in 

 which the organism commenced its existence. The more ex- 

 tensively diffused such cells were, the more generalh' might 

 multiplication by budding or fission take place ; the more lo- 

 calized, the more limited would be the parts of the org-anism 

 in which such a process would take place. And, even w^here 

 such cells occurred, their development or non-development 

 might be connected with conditions of nutrition. It depends 

 en the nutriment supplied to the female larva of a bee wheth- 

 er it shall become a neuter or a sexually perfect female ; and 

 the sexual perfection of a large proportion of the internal 

 parasites is similarly dependent upon their food, and perhaps 

 on other conditions, such as the temperature of the medium 

 in which they live. Thus the gradual disappearance of aga- 

 mo2:enesis in the hig-her animals would be related witli that 

 increasing specialization of function which is their essential 

 characteristic ; and, when it ceases to occur altogether, it 

 may be supposed that no cells are left which retain unmodified 

 the powers of the primitive embryo cell. The organism is 

 like a society in which every one is so engrossed by his spe- 

 cial business that he has neither time nor inclination to marrv. 

 Even the female elements in the highest organisms, little 

 as they differ to all appearance from undifferentiated cells, 

 and though they are directly derived from epithelial cells 

 which have undergone very little modification from the condi- 

 tion of blastomeres, are incapable of full development unless 

 they are subjected to the influence of the male element, which 

 may, as Caspar Wolff suggested, be compared to a kind of 



