ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 195 



The difference is referable, first of all, to the more or less extensive 

 ^accumulation of nutritive yolk, the significance of which for the 

 nascent organism is twofold. 



From a physiological point of view, the nutritive yolk is a rich 

 source of energy which alone makes it possible for the embryologicul 

 processes to take place in uninterrupted sequence, until at length an 

 organism, with an already relatively high organisation, begins its 

 independent existence. 



From a morphological point of view, on the other hand, the yolk plays 

 the role of ballast, which exerts a restrictive and modifying influence 

 on the direct and free development of those organs which are en- 

 trusted with the reception and elaboration of it. Even at the very 

 beginning of development we could see how the cleavage- process and 

 the formation of the germ -layers were retarded, altered, and to a certain 

 extent even suppressed by the presence of yolk. In what follows we 

 shall again have occasion to point out the same thing, how, owing 

 to the presence of yolk, the normal formation of the intestinal canal 

 and of the body can be attained only gradually and by a circuitous 

 process. 



In the second place, the great difference which the embryos of 

 Vertebrates present is produced by the medium in which the eggs 

 undergo development. Eggs which, like those of water- inhabiting 

 Vertebrates, are deposited in the water, are developed in a more 

 simple and direct manner than those which, provided with a firm 

 shell, are laid upon the land, or than those which are enclosed in 

 the womb up to the time of the birth of the embryos. 



In the two latter cases the growing organism attains its goal only 

 by very indirect ways. At the same time with the permanent organs 

 there are also developed others which have no significance for the 

 post-embryonic life, but which serve during the egg-stage of exist- 

 ence either for the protection of the soft, delicate, and easily injured 

 body, or for respiration, or for nutrition. These either undergo 

 regressive metamorphosis at the end of embryonic life, or are cast 

 oft' at birth as useless and unimportant structures. But inasmuch as 

 they are developed out of the germ-layers, they are also properly to 

 be regarded as belonging immediately to the nascent organism as 

 being its embryonic organs, and as such they too are to be treated in 

 morphological descriptions. 



The extensive material which has to be mastered in this con- 

 nection I shall present grouped into two parts. 



In the first part we shall inquire how the embryo overcomes the 



