458 BERTRAM G. SMITH 



by preservation in formalin, gives the best photographic results. 

 While being photographed, the objects were immersed in water 

 or formalin. Living larvae were anesthetized with chloretone. 

 In all cases the exposure was made by daylight. 



All the photographs with a magnification of X 4 were made 

 with a Bausch and Lomb Zeiss Tessar 72 mm. lens, fitted to a 

 long bellows Pony Premo No. 6 camera. The camera was fas- 

 tened in an erect position by means of an improvised wooden 

 frame. Figures 262, 264, 265 and 266 were taken with a Zeiss 

 Unar Lens; figures 263 and 267 with a Zeiss Apochromatic Planar. 

 Seed's Non-halation plates were used throughout; they were 

 developed with Adurol. 



All the negatives are the work of the author except figure 266 

 which was made by Miss Frances J. Dunbar of the University 

 of Michigan. 



The photographs are untouched, except in a very few cases 

 for the purpose of correcting slight defects in the negatives. 



VII. CLEAVAGE 

 A . Description of cleavage by stages 



Stage 1: {figs. 57 to 61 and 204). This stage is characterized 

 by the presence of the first cleavage furrow only. The germinal 

 area reaches nearly to the equator. 



In artifically fertilized eggs the first cleavage furrow ordinarily 

 appears about twenty-four hours after fertilization; the time 

 may vary from eighteen to twenty-eight hours. The furrow 

 begins as a pit, usually at the animal pole; it lengthens rapidly 

 at first, then more slowly as it invades the regions of the egg more 

 heavily laden with yolk. After the first cleavage furrow is well 

 established, it becomes narrow in its middle portion, while still 

 broad at the ends. The first cleavage furrow of the typical form 

 becomes superficially complete in Stage 3 (third cleavage). 



In the typical condition, the first cleavage furrow passes through 

 the animal pole, and the division is equal (figs. 57 and 204). 

 Variations from this condition occur. To test the amount of 



