HYDRA 53 



9. GASTRULA STAGE. In optical section show relative 

 thickness of ectoderm and of entodenn. Measure diameter and 

 compare with that of immature egg. 



10. Write a connected account of early development in 

 Cerebratulus, bringing into relationship the processes of matura- 

 tion, fertilization, mitosis, and cleavage. In describing the first 

 cleavage give in full an account of the process of mitosis as it 

 occurs. 



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 pilidium. The mature worm, 

 several feet in length, results from the further growth and com- 

 plicated transformation of this larva. 



REFERENCE 



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

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



HYDRA 



(Materials: thread, filter paper, methylen blue.) 



Classification : Phylum Coe'lenterata, Class Hydrozoa, Or- 

 der Hydraria. 



1. LIVING HYDRA. A living specimen will be given you in a 

 watch glass with a small amount of water. Examine with dis- 

 secting lens while still in the watch glass, using care in handling 

 the specimen to prevent injuring it. 



2. GENERAL STUDY. Note: (i) Body, size and form. Does 

 the form change? (2) The foot or part by which it becomes 

 attached. How do you infer that attachment is effected? See 

 demonstration of longitudinal section through foot. Also examine 

 living specimens clinging to sides of an aquarium. (3) The 

 tentacles. How many? Where situated? Change of form? (4) 



