258 W. JR. Coe — Development of the 



SurroundiDg the whole alimentary canal and connected with the 

 other muscular bands by frequent anastomoses is an irregular net- 

 work of scattered fibers which provide this organ with independent 

 motion and also serve to hold it in place. 



Sheffield Biological Laboratory of Tale University. 



EXPLANATION OF PLATES. 

 Plate 2^X1. 



Early cleavage of the egg of Micrura cceca. Camera lucida drawings of living eggs 

 at a magnification of about 300 diameters. 



Figure 1. — Egg about one hour and forty minutes after fertilization. The fully formed 

 spindles of the second cleavage are indicated by dotted lines. Above are the two 

 polar bodies. 



Figure 2. — The same egg eight minutes later. The spindles are still evident. The 

 polar bodies he between the blastomeres. 



Figure 3. — The same egg after fifteen minutes more. The blastomeres have drawn 

 very closely together, leaving but a minute cavity between them. 1, 1, denote 

 the plane of the first cleavage ; 2, 2, that of the second cleavage. A, B, C, D, 

 the four quadrants of the egg. One of the asters of the third cleavage is seen 

 in each blastomere. 



Figure 4. — The third cleavage just completed. The upper four cells are widely sepa- 

 rated. Each lies somewhat outside and to the right (following the hands of a 

 watch) of its lister-cell. 



Figure 5. — Eight-celled stage from the side. The first polar body lies above. 3, 3, 

 denotd the plane of the third cleavage. 



Figure 6. — Eight-celled stage from above after the blastomeres have drawn together. 

 Same position as fig. 3. Polar bodies are not shown. 



Figure 7. — Sixteen-celled stage from above. In the same position as fig. 3. 



Figure 8. — The same stage from the side. 



Plate XXXII. 

 The mesoderm cells are indicated in red. 



Figure 1. — Vertical section of the blastula oi Micrura coeca, nine hours after fertiliza- 

 tion of the egg. The two large cells (one of which is in the process of division) 

 within the segmentation cavity are mesoderm cells, x 600. 



Figure 2. — Horizontal section of the gastrula of M. coRca, at a stage somewhat later 

 than that shown in fig. 3. The section passes along a plane at a level with a;, x, 

 fig. 1, PI. XXXIII. The circle of ciliated cells in the center represents a section of 

 the entodermic sack. In the space between the entoderm and ectoderm are seen 

 several cells of the larval mesoderm. These cells are now much smaller and 

 more numerous than in fig. 1. x 600. 



