2io AGE, GROWTH, AND DEATH 



The eyes of crabs and related animals (Decapoda) 

 are born on stalks. If these stalks are cut partly off so 

 as to remove all of the eye, but leave part of the 

 nervous centre {ganglion) in the base of the stalk, 

 it is found that in a considerable percentage of cases a 

 new eye will be formed. Miss M. I. Steele 1 has made 

 a study of this regeneration. She states that the 

 healing over of the wound and the growth of the 

 undifferentiated cells of the outer layer (ectoderm 

 or hypodermis) occurs much as just described for 

 the antenna of Oniscus. Later the hypodermal cells 

 lose their primitive and simple character and by 

 passing through various but co-ordinated differen- 

 tiations evolve new complete ommatidia, as the 

 structural units are termed which constitute the eye 

 in crabs and other Crustacea. It may interest you to 

 know that if the whole or nearly the whole eye-stalk 

 be cut off, regeneration may still take place, but if it 

 does a feeler-like appendage or antenna results, with- 

 out any special optic apparatus. This singular phe- 

 nomenon was first observed by Professor Herbst. 2 



One of the interesting illustrations of the import- 

 ance of undifferentiated cells is afforded, according to 

 recent observations, by certain worms of lowly organ- 

 isation, known as planarians. The space between the 

 organs of one of these animals is occupied by a low 

 form of connective tissue, a syncytium termed the 



1 Mary Isabella Steele, "Regeneration in Compound Eyes of Crustacea," 

 Jour. Exp. Zool., v., 163-244, 16 pis. (1907). 



2 C. Herbst, Arch. f. Entwickelungsmec/wnik, ii., 455-516(1896). See also 

 Bd. ix., 215-293 (1900). 



