BLASTULA, GASTRULA, AND GERM LAYERS 363 



gradually disappear, either wholly (tail of tadpole), or in part 

 (pronephros) remaining then as vestigial organs. 



Changes in cell form and size chiefly fall under the head of tissue 

 differentiation, or histogenesis, but in a few instances such processes are 

 primarily involved in the formation of rudiments. The development 

 of the eye affords illustrations of these processes; the lens forms as a 

 thickening of the cells on one side, and the thinning on the other, of 

 a sac originally formed by invagination, and the optic lobe, at first 

 nearly spherical, flattens and invaginates, one layer becoming thickened 

 as the chief part of the retina (later complicated by cell proliferation) 

 while the other layer forms a thin pigmented layer. 



These processes of morphogenetic value are tabulated in the accom- 

 panying summary; the arrangement here is merely a convenient one 

 and has no other significance. 



It is extremely important to recognize that these morphogenetic 

 processes described above are not merely simple mechanical processes. 

 The arrangement and behavior of the cells in an invaginating or delami- 

 nating region are determined by other factors than those of physical 

 resistances, attractions, etc. These events are both in fundamentals 

 and in details to be regarded as active phenomena of a living organism. 

 They are frequently also to be understood from the historical or adaptive 

 points of view. 



What the precise conditions are which determine the nature of 

 these events, may usually only be conjectured. In some cases they may 

 result from osmotic conditions, absorption of water, etc. But for the 

 most part the nature of the specific stimuli, and the conditions within 

 the layer or cell group, which lead to the definite reactions of rudiment 

 formation are unknown. And in this particular field of development we 

 can do little more than to describe what happens from the morphological 

 viewpoint. 



