EMBEYOLOGY 57 



undergoes the same stages outlined in the preceding paragraph. 

 Make three drawings to show these stages in the second cleavage. 

 Figs. 13 to 15. 



6. Third Cleavage. Draw a polar and a lateral view. In 

 this stage note that the four cells at the animal pole are not 

 directly above the four cells of the vegetative pole. This shift- 

 ing of the position of the cells indicates what is called the ' spiral 

 type of cleavage.' Figs. 16 and 17. 



7. Fourth Ci eavage. Draw a polar view. By focusing 

 note that a cavity is already beginning to form in the center of 

 the mass of cells. In Cerebratulus there is no morula stage. 

 Fig. 18. 



8. Blastula Stage. Draw in optical section showing the 

 blastula cavity or blastocoel which is surrounded by a single 

 layer of cells. Fig. 19. 



9. Gastrula Stage. In optical section show relative 

 thickness of ectoderm (outer cell layer) and entoderm (inner 

 cell layer. The cavity between them is the hlastocoel. The 

 cavity within the entoderm layer is termed the gastrula cavity 

 or archenteron. It opens to the exterior through an opening, 

 the blastopore. Determine the diameter of entire gastrula and 

 compare with that of immature egg. Draw as Fig. 20. 



10. During the gastrula stage the Cerebratulus is a free- 

 living animal. By further development the gastrula becomes 

 modified to form a small larva known as a pilidimn. The mature 

 worm, several feet in length, results from the further growth 

 and complicated transformation of this larva. 



. 11. Write a connected account of early development in 

 Cerebratulus, bringing into relationship the processes of matura- 

 tion of the egg, fertilization, mitosis, and cleavage. In describ- 

 ing the first cleavage, give in full an account of the process of 

 mitosis as it occurs in this animal. 



REFERENCE 



Wilson, E. B., 1903. Experiment on Cleavage and Localization 

 in the Nemertine-egg. Arch. Entw.-mech. 16 :411-460. 



