REPRODUCTION AND DEVELOPMENT 51 



a safety-razor blade. Several trials may be necessary but remarkably 

 good preparations are often secured by this rough method. Study 

 under water with the handlens and the low-power objective of the 

 compound microscope with the mirror turned aside. What is the 

 extent of the blastula cavity? Has it an external opening? What is 

 the size of the cells? Have the cells begun to divide in planes parallel 

 to the surface of the blastula? Draw the cut surface of the half blas- 

 tula. Examine a demonstration section of this stage. 



Exercise 52. — The Late Blastula. 



(b) Examine older blastulse in which the surface cells have become 

 much smaller. Is there any sign of differentiation anywhere on the 

 spherical mass, other than in the distribution of pigment? Draw from 

 a side view, showing the size of the cells by making a few cell outlines 

 in each hemisphere. 



E. The Gastrula Stage 



Exercise 53. — The Early Gastrula. 



(a) Examine a stage several hours older than the last, using hand- 

 lens and low-power objective. Can you recognize cell outlines at each 

 pole? If a specimen is not found with the vegetal pole up, one can be 

 secured as follows: cut a piece of filter paper a centimeter square and 

 lay upon a slide; add enough water to saturate the paper and hold it 

 in place but not enough to float it; using another piece of the paper, 

 roll off the jelly from a single specimen and transfer to the above slide. 

 The specimen can then be rolled into the desired position. If details 

 are not clear under the low-power objective, add a cover glass, sup- 

 ported on one side by a pin, and run water under the cover glass with 

 a pipette. Examine the vegetal hemisphere and the equatorial region. 

 At this stage the pigmented cells of the animal pole begin to encroach 

 upon the surface area of the lower, yolk-laden cells. This is the result 

 of movement of cells following the onset of a more rapid rate of cell 

 division in the cells of the equatorial region, which are now known as 

 the germ ring. This movement of the cells of the germ ring is soon 

 accompanied by an inturning of cells, the position of which can be seen 

 on one side of the equatorial region as a crescent-shaped line separat- 

 ing the light and dark cells. The crescent-shaped line between the 

 light and dark cells is really a narrow slit, the blastopore, which is the 

 external opening of the gastrula cavity, or archenteron. The pig- 

 mented cells that border the blastopore constitute the dorsal lip of the 

 blastopore; they are cells of the germ ring. Draw the early gastrula 

 as seen from the vegetal hemisphere. 



