PROTOZOA TO WORMS 45 



layer of " white " represents the entoderm or lining of 

 larger cells. The space occupied by the yolk corre- 

 sponds to the archenteron or primitive digestive cav- 

 ity ; and the opening at the end to the primitive mouth 

 or blastopore. Ectoderm and entoderm unite around 

 the mputh. Both the blastosphere and gastrula often 

 swim freely by flagella. 



You can hardly have failed to notice how closely the 

 gastrula corresponds to a hydra, and many facts lead 

 us to believe that the still earlier ancestor of the hydra 

 was free swimming, and that the tentacles are a later 

 development correlated with its adult sessile life. Yet 

 we must not forget that the hydra is even now not 

 quite sessile, it moves somewhat. And our ancestor 

 Avas almost certainly a free swimming gastraea, or hy- 

 pothetical form corresponding in form and structure to* 

 the gastrula. The ancestor of man never settled down 

 lazily into a sessile life. 



But how is an adult worm or vertejsrate formed out 

 of such a gastrula ? To answer this would require a 

 course of lectures on embryology. But certain changes 

 interest us. Between the ectoderm and entoderm of 

 the gastrula, in the space occupied by the supporting 

 membrane of hydra, a new layer of cells, the meso- 

 derm, appears. This has been produced by the rapid 

 growth and reproduction of certain cells of the ento- 

 derm which have migrated, so to speak, into this new 

 position. In higher forms it becomes of continually 

 greater importance, until finally nearly all the organs of 

 the body develop from it. In our bodies only the lining 

 of the mid-intestine and of its glands has arisen from 

 the entoderm. And only the epidermis, or outer layer 

 of our skin, and the nervous system and parts of our 



