370 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



accessory functions, such as the procuring of food, the distribution 

 of food within the body, and the elimination of waste products of 

 metabolism. The necessity of protection and the reproductive func- 

 tion have, it is true, had their influence on bodily form, but their 

 effects are not so far-reaching or so deeply stamped in the body struc- 

 ture as are the characters that pertain directly or indirectly to the 

 function of nutrition. 



It is probable that in a very early stage in the evolution of the 

 Metazoa the animal consisted of a mere sphere of cells, but that the 

 cells later became arranged in a single layer on the outside, forming 

 thus a cellular sac filled with liquid. The embryonic representative 

 of this evolutionary stage is called a hlastula (fig. 4 A). Its outer 



Bid 



A_ 



B 



^F 



FlQUBB 4.- 



-Diagrams of early stages of general embryonic development, and the forma- 

 tion of the first stomach, or archenteron. 



A, the blastula. B, dififerentiation of the digestive cells of the bastula. C, the gastrula, 



produced by invagination of the digestive cells to form an endodermal stomach (End). 



Bio, blastocoele ; Bid, blastoderm ; Bpr, blastopore ; F, food material ; Oc, gastrocoele ; 

 He, haemocoole ; Ecd, ectoderm ; End, endoderm. 



cell laj^er is the Nastoderm {Bid.). In this stage the animal has no 

 true stomach, and it cannot be said just how a free-living creature 

 in the blastula condition obtained its food. Perhaps, however, all 

 its cells were individually ingestive and digestive. 



If embryonic development may be taken in a general way as a 

 guide to the history of evolution, it becomes apparent that the 

 blastula soon concentrated its digestive and absorptive functions in 

 the cells of one particular area of its surface, probably the area most 

 likely to come into contact with food material, for in the next stage 

 of embryonic development it is usuallj^ found that the cells of one 

 side of the blastula have become larger than the others (fig. 4 B), 

 and subsequent history shows that these cells form the digestive 

 surface of the animal. In the blastula this surface is often somewhat 

 flattened or slightly concave. The ancestral representative of the 

 blastula almost certainly inhabited water, and, if it lived mostly on 

 submerged surfaces, it is much to be suspected that, when hungry, 

 it simply sat down on any convenient mass of food material (F) 

 and consumed it. Whether it practiced internal or external diges- 

 tion we cannot pretend to decide, but in any case the digestion 



