312 Tin: KVOLUTIOX OF MAN. 



Coiiiparaf ive Anatomy, like every other science, is enfllesa ; anrl (I)Prpfore 

 the eudle.i.-iuoss of tht; accinniilntion of materials would never allow men, if 

 they complied with this demniid, to reap any harvest from this field. But, 

 further than ihis, history teaches us clearly, tliat no age in which scientific 

 inquiry has been active, has been able so to deuy itself, as, setting the goal 

 of its researches in the future, to refrain from drawing conclusions for itself 

 from its larger or smaller treasury of observations, and from trying to fill th( 

 gaps with hypotheses. It would, indeed, be a hopeless proceeding, if, in 

 order to avoid losing any part of our possessions, we should refuse tc 

 acquire any possessions whatever." — Karl Ernst Raer (1819). 



Among the vegetative organs of the human body, to tlie 

 development of which we now turn our attention, the intes- 

 tinal canal is the most important. For the intestinal tube 

 is the oldest of all the organs of the animal body, and 

 cari-ies us back to the earliest time of or<:raiiolo2:ical differ- 

 entiation, to the first period of the Laurentian Epoch. As 

 we liave alreadv seen, the result of the first division of 

 labour in the homogeneous cells of the earliest many-celled 

 animal body must have been the formation of a nutritive 

 intestinal canal. The first duty and the first need of every 

 organism is self-support. This task is accomplished by the 

 two functions of nutrition and of the covering of the body. 

 When, thei'efore, in the primaeval collection of homogeneous 

 cells {Synamcehium), of the phylogenetic existence of which 

 we yet have evidence in the ontogenetic developmejital 

 form of the mulberry-germ (Morula), the several members 

 of the community began to divide the work of life, they 

 were fiist obliged to engage in tAvo separate tasks. One 

 h.-df modified into nutritive cells, enclosinix a diofestive 

 cavity, the intestinal canal ; the other half, on the contrary, 

 developed into covering cells, forming the outer cover- 

 ing of this intestinal canal, and, at the same time, of tlic 

 whole body. Thus arose the first two gurm-layers : the 



