2l6 EDWIN G. CONKLIN. 



photograph, but its anterior end appears as a clear, triangular 

 area, notched where the tube is still open at the anterior end of 

 the larva. This clear, transparent condition of the nervous sys- 

 tem, both in the tail and trunk regions shows that the yellow 

 protoplasm does not enter into its formation and that the muscle 

 cells are not " neuro-muscular " cells as claimed by Castle (1896). 

 In this and the following photograph (Photo 24) the larva is 

 well developed, though the sense organs have not yet appeared 

 in the sense vesicle. The most important organs of the larva 

 are here clearly recognizable in the photographs of the living 

 tadpoles, viz., the muscles, the notochord, the central nervous 

 system, the gastral endoderm, the caudal and trunk mesen- 

 chyme and the ectoderm. T/ic substance of each of these organs 

 is peculiar in color and constitution and tJiesc different substances 

 may all be traced back to the 2-cell stage, where they occupy 

 positions corresponding to tlicir ultimate locations in the larra, while 

 the substances of the ectoderm mcsodcrm and endoderm are recog- 

 nizable in the unsegmcnted egg. With the exception of the early 

 gastrula stages, which were not photographed, every important 

 step in the transformation of these substances into the organs 

 named can be followed in the photographs of the living eggs and 

 embryos ! 



NATURE AND POTENCY OF THE OOPLASMIC SUBSTANCES. 



The fact that definite blastomeres of the ascidian egg give 

 rise to definite portions of the larva has long been known (Van 

 Beneden and Julin, 1884; Castle, 1896). Furthermore Chabry 

 (1887) found that when certain blastomeres were killed the re- 

 maining ones gave rise only to a partial larva. On the other 

 hand, Driesch (1895, 1903) and Crampton (1897) found that in- 

 dividual blastomeres of the ascidian egg developed into entire 

 larvae. The mere observation of the egg of Cynthia shows that 

 certain areas are marked out from the time of fertilization, or 

 even earlier, by distinct kinds of protoplasm and that these areas 

 give rise in the course of normal development to definite organs. 

 But, in view of the work of Driesch and Crampton, by what 

 right are these areas called organ-forming regions and what is 

 the justification for calling the substances of these areas organ- 



