522 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



supported on pillars. Sometimes such large elements rest upon 

 the floor of the cavity without being in contact with the blasto- 

 derm above; in such cases it must remain doubtful whether 

 these elements have been detached during life, or have been 

 separated by the action of reagents. The presence of such 

 variously sized elements lying on the floor of the cavity is 

 however so constant, that it is difficult to avoid the conclusion 

 that they have been separated during life, or have remained 

 adherent to the floor of the cavity when the blastoderm has 

 been raised from it. 



Near the free border the blastoderm is thickened, chiefly 

 owing to the large cleavage spheroids situated upon its lower 

 surface, but beyond the thickening the border becomes thin 

 and sharp. It rests with the thickening upon the yolk, and 

 thus lies like a cover over a shallow depression in the latter. 



Fig. 403. 



Fig. 403. Section through the germ of a fresh-laid egg in the month 

 of June. 



The cavity which is thus formed was termed by Remak the 

 "Germ-cavity" (Keimhole). The wall bounding the cavity 

 named by His the " Germ-wall" (Keimwall). This wall, how- 

 ever, is by no means a constituent of the germ ; it belongs to 

 the yolk, and the term germ- wall is consequently inappropriate. 

 As the accounts of the layers of the embryo are associated 

 with the last-mentioned stage, this is the place in which a 

 sketch of the literature bearing upon the subject may be 

 introduced. 



We are indebted to Caspar Friedrich Wolff* for the prevailing 

 doctrine respecting the development of the Vertebrata from the so-called 



* De formatione intestinorum, 1768-69. Translated into German by 

 Meckel, 1812. 



