1918 Osterud: on Embryologi/ of Leptasterias 5 



The appearance of the young starfish when it leaves the mother is 

 shown in figures 16 and 17. The need of self-protection is met by the 

 very rapid growth of numerous and very prominent spines and of the 

 skeleton. After the young escape they disappear very quickly from the 

 habitat of their mothers. In spite of their prolonged protection and ad- 

 vanced development the mortality must be very great. A few specimens 

 can usually be found on the eel grass (Zostera) near by, where, by July 

 first, they have reached a size of 3 to t mm. 



The interest in this starfish is mainly in connection with the 6th ray, 

 iji the development of which, at least as far as superficial results are con- 

 cerned, it corresponds nicely with what Gemmill (1912) found in Solaster 

 cndeca. The writer does not believe that the delay in the development of 

 the 6th ray is due entirely to interference by the preoral lobe, altho its 

 development after it does begin is undoubtedly retarded by this organ. But 

 the initial appearance of 5 similarly developed hydrocoele pouches is in- 

 terpreted as due to the persistence of a fundamental racial pentamerism, 

 upon which the later hexamerism is superimposed. In other words, Lep- 

 tasterias exhibits the first complete step in the addition of a whole new unit 

 of structure, which Gemmill has so w'ell shown, is carried further in a 

 similar manner in Solaster enc^eca. It represents the individuation of a 

 new part from the embryonic tissues. The fact that a o-rayed or a 

 7-rayed form is occasionally found would lead one to surmise that it may be 

 possible to control this individuation experimentally. It is hoped that 

 the opportunity will present itself to some one to make a test of the possi- 

 bilities along this line; also that the internal details of development may 

 some day be checked with those of other starfishes. 



LITERATURE CONSULTED 

 Child, C. M. 



1915. Senescence and rejuvenescence. University of Chicago Press. 

 Gemmill, J. F. 



1912, The development of the starfish, Solaster endeca (Forbes). 

 Trans. Zool. Soc. London, Vol. 20. 

 IVIacBride, E. \V. 



1896 The development of Asterina gibbosa. Quart. Journ. ]\Iic. 



Soc, Vol. 38. 

 191 i. Textbook of embryology. Vol. 1. 

 ISIasterman, A. T. 



1902. The early development of Cribrella oculata, with remarks on 

 echinoderm development. Trans. Roy. Soc. Edinburgh, 

 Vol. 10. 

 Verrill, E. A. 



1914. Monogra|)h of tlie shallow water starfishes of the north Pacific 

 Coast from the Arctic Ocean to California. Reports of the 

 Harriman Alaska expedition. Vol. 14. 



