THE EVIDENCE FROM DEVELOPMENT. 



185 



tween the development of widely different animals ends not thus. If 

 the process of segmentation is universal, the morula or " mulberry 

 stage/' in which that process culminates, is seen to be no less uniform 

 and unvarying in its occurrence. Even among the Protozoa, as 

 we have already remarked, we may perceive stages (Fig. 98, 6) in 

 development which imitate the " mulberry mass " of higher forms. 

 We have already traced the occurrence of this stage in the sponge 

 and in the other life-histories described in this chapter ; whilst 

 a wider survey of the animal world would serve to show that in the 

 early history of every group the " mulberry stage" is to be witnessed, 

 as the first prominent landmark or halting-place on the journey of 

 life. The egg of such an animal as a " Tardigrade " or Bear-animal- 

 cule minute organisms allied to the mites, and found in the gutters 

 of house-tops thus exhibits in its development stages of a nature 

 essentially similar to those seen in the history of both lower and 

 higher forms of animal life. The egg itself (Fig. 99, i) exhibits a 

 structure comparable with that of all other germs. In its develop- 

 ment the germ not only passes through the stages of segmentation 

 (2, 3) already familiar to us in the sponges, sea-squirts, and vertebrates, 

 but also arrives in due course at the mulberry epoch (4), wc'thorula, 

 whence the special fea- 

 tures of the Tardigrades 

 are specialised. How per- 

 fectly these details in the 

 animalcule correspond 

 with the stages in the 

 development of the verte- 

 brates or highest animals, 

 is a fact which may be best appreciated by the comparison' of the 

 segmentation of the egg of a vertebrate animal depicted in Fig. TOO, 

 from its commencing development (i) to the attainment of the mul- 

 berry stage (5); whilst that of the frog (Fig, 101) exhibits essentially 

 the same phases as the developing germ -of bird or mammal. With 



FIG. 99. DEVELOPMENT OF BEAR-ANIMALCULE. 



FIG. ioo. SEGMENTATION OF VERTEBRATE EGG. 



Professor Allen Thomson we may therefore hold that "the occur- 

 rence of segmentation and the regularity of its phenomena are so 

 constant, that we may regard it as one of the best established series 

 of facts in organic nature." 



