EMBRYOLOGY 63 



ship to the structures already noted and to each 

 other? Note that the furrows do not form a right 

 angle at the apical pole; the small cross furrow is 

 called the polar furrow. How is it produced? 



332. The third cleavage (eight-cell stage). Note the 

 third, meridional, cleavage furrow. What relation- 

 ship has it to the first two and to the hemispheres? 

 Which of the cells (blastomeres) are the larger? 



333. The fourth cleavage (twelve- and sixteen-cell 

 stages). Note that the fourth cleavage is produced 

 by two furrows. What is their relationship to each 

 other, and to the other cleavage furrows? How can 

 you account for the existence of the twelve-cell 

 stage? 



334. The fifth cleavage (twenty-four and thirty-two cell 

 stages). The fifth cleavage is also produced by two 

 cleavage furrows. How does their relationship to 

 each other differ from that of the two furrows in the 

 fourth cleavage? How can you account for the 

 existence of a twenty-four cell stage? Are the 

 blastomeres equal in size and regular in arrangement, 

 or not? 



335. The blastula. Note the small size of the cells. 

 Hemisect through the apical pole and note the 

 cavity (blastoccele). Examine under binocular mi- 

 croscope if possible. If blastulae are not available, 

 this exercise can be performed on the fifth cleavage 

 stage (334). The cavity is then called the segmen- 

 tation cavity. 



336. A late gastrula (yolk-plug stage). Note the small 

 white protuberant yolk-plug, which occupies the 

 potential blastopore. All the rest of the surface is 

 composed of minute pigmented cells, the ectoderm. 

 Hemisect the gastrula through the yolk-plug and 



