No. 3-] THE EMBRYOLOGY OF A TERMITE. 543 



first sight. Here was another case of the nearest explanation 

 not necessarily being the true one. 



It seems hardly necessary to say that the fact that such 

 invaginations can be watched step by step sometimes, and can 

 be actually observed to encounter resistance at every stage, 

 is no proof that such resistance causes the process. 



a. 3. My study of the formation of the embryonic rudiment 

 and of the origin of the amniotic fold of the Termite indicates 

 forces of a very different nature from those formulated by 

 Wheeler; in fact the very reverse. 



As the germ-disc becomes sharply defined, the area of the 

 blastoderm occupied by it is distinguished by the closer crowd- 

 ing of its cells, while the surrounding cells become flattened 

 and pulled apart into a thin membrane. There appears to 

 be a contraction toward the embryonic area, as is observed 

 in the formation of the embryonic rudiment in other insects 

 and other arthropods. At any rate, the extra-embryonal blas- 

 toderm may be said to be stretched and kept so by the changes 

 taking place in the embryonic area. 



Before the amnion arises it is clearly differentiated as a 

 special thickened area of the germ-disc. When the embryonic 

 rudiment doubles-up, and this posterior portion of it folds over 

 to become the amnion, the extra-embryonal blastoderm is 

 pulled forward and further stretched. 



// seems correct to speak of the tension of the serosa as due 

 to the activities in the embryonic area, ratJier than to reverse 

 the case and explain important changes in this area as a result 

 of sucJi tension. 



In studying the growth of the germ-disc, I can find no indi- 

 cations of a rigid resistance to its growing edges claimed to 

 be offered by the rest of the blastoderm. The cells around 

 the rapidly proliferating area do not seem to be fixed, im- 

 movable points ; and the membrane they form does not 

 appear to be more resistant to this more active area than 

 is the yolk. 



Another important point is the fact, as I have shown, that 

 the amnion is not a derivative of the extra-embryonal blasto- 

 derm, as Wheeler (26) concludes in his latest paper. 



