46 THE WHENCE AND THE WHITHER OF MAN 



sense-organs have arisen from the ectoderm. But our 

 mid-intestine is still the greatly elongated archenteron 

 of the gastrula. 



We may therefore compare the hydra or gastrula 

 to a little portion of the lining of the human mid-in- 

 testine covered with a little flake of epidermis. This 

 much the hydra has attained. But our bones and 

 muscles and blood-vessels all come from the mesoderm 

 by folding, plaiting, and channelling, and division of 

 labor resulting in differentiation of structure. Of all 

 true mesodermal structures the hydra has actually 

 none, but in the ectodermal and entodermal cells he 

 has the potentiality of them all. We must now try to 

 discover how these potentialities became actualities in 

 higher forms. 



The third stage in our ancestral series is the tur- 

 bellarian. This is a little, flat, oval worm, varying 

 greatly in size in different species, and found both in 

 fresh and salt water. Some would deny that this worm 

 belonged in our series at all. But, while doubtless 

 considerably modified, it has still retained many char- 

 acteristics almost certainly possessed by our primitive 

 bilateral ancestor. The different parts of hydra were 

 arranged like those of most flowers, around one main 

 vertical axis ; it was thus radiate in structure, having 

 neither front nor rear, right nor left side. But our 

 little turbellaria, while still without a head, has one 

 end which goes first and can be called the front end. 

 The upper or dorsal surface is usually more colored 

 with pigment cells than the lower or ventral surface, on 

 which is the mouth. It has also a right and left side. 

 It is thus bilateral. 



The gastrtea swam by cilia, little eyelash - like 



