148 THE DEVELOPMENT OP 



egg the better, and hence any modification which would 

 place each organ in its proper place at first, without the 

 aid of later interstitial growth, would be a material gain. 

 In this way the two extremities of the body would come 

 to lie at the two poles 1 of the egg (compare fig. 10 or bet- 

 ter, Mayer, ('77, pi. xiii, fig. 15), while the appendages 

 would arise between. 



An increase in the amount of food-yolk would result in 

 an increase in the size of the egg, and, supposing the first 

 formed rudiments to retain their relative positions, this 

 would of course widely separate the organs first to ap- 

 pear. Now, we may imagine it would be an economy for 

 the embryo, in its early stages, when the protoplasm was 

 scanty in comparison with the food-yolk, to have its parts 

 near together, and it may be that in this way this strange 

 contraction of the germ has been introduced. In other 

 words the widely-separated, optic lobes and thoracico-ab- 

 dominal area of stage A are an inheritance from a small 

 egged precocious ancestor, while the contraction seen in 

 the later stages is a consequent of the increase in amount 

 of food-yolk. 



The manner in which this contraction is produced is al- 

 most equally obscure. I regret that I have made no accu- 

 rate measurements on the living egg which might throw 

 light upon it. For the present I accept the explanation 

 of Mayer ('77, p. 232. " Offenbar kommt eine solche 

 Naherung aller einzelnen Partien auf der Bauchseite des 

 Embryo nur dadurch zu Stande, dass sich die zwischen 

 ihnen liegenden Blastodermzellen contrahiren und sich 

 hiermit zugleich in ein Cylinderepithel umwandeln." 

 This of course involves a corresponding expansion of the 

 blastoderm of the dorsal surface, and Mayer calls for the 



1 The term pole is used with a mathematical and not with a physiological sig- 

 nificance. 



