LAEVAE OF ARBACIA PUNCTULATA 321 



calcium was replaced by sodium. They found that the latter 

 medium not only produced lack of skeleton, but also diminished 

 rate of growth, decreased size, and lowered vitality, so that a 

 much smaller percentage reached the gastrula stage than when 

 normal sea- water was employed. 



Herbst ('93), investigating the effects of changed composition 

 of the sea-water upon development, concluded that the modifi- 

 cations produced in this manner are brought about as the result, 

 not of chemical but of physical changes, especially of osmotic 

 pressure. He states that the effect of like amounts of differ- 

 ent salts decreases with increasing molecular weights of the 

 latter; but this law holds only for the equi-basic salts of one 

 and the same metal. To the salts of different metals, tissues are 

 permeable in varying degrees; that is, the epithelium of the 

 Echinoid larva is almost negligibly permeable to lithium, but 

 somewhat readily to potassium and sodium. He believed ''dass 

 die Seeigel lar^'en den Salzen des umgebenden Mediums gegen- 

 iiber ein gewisses Auswahlvermogen geltend machen and nur 

 solche Stoffe aus dem Meerwasser aufnehmen, welche fiir ihre 

 Entwicklung notwendig sind." 



In a later paper ('95), however, he altered his views in regard 

 to the cause of modifications produced by lithium salts and 

 concluded that the effects could be of a chemical as well as 

 of a physical nature; — ''dass die Zellen der Echinidenlarven 

 die Salze des betreffenden Metallen aufnehmen .... und 

 zurtickhalten." 



Still later ('95), he carries further his studies of the effects of 

 changes of the medium, employing various salts of lithium and 

 sodium. The results of these experiments show, he says, "dass 

 man mit sammtlichen Lithiumsalzen den normalen Entwickel- 

 ungsgang in derselben Weise wiirde abiindern konnen, wenn die 

 Saure, an welche das Metall gebunden ist, nicht schadlich auf 

 die Eier resp. Larven wirken wiirde. An dem Zustand der 

 Zellen jener Blastulae, welche in einer Mischung von Lithium 

 aceticum gestorben sind, sieht man, dass dieselben durch 

 Essigsaure abgetodtet worden sind. Es scheint hiernacht, als 

 wiirde das organische Lithium-salze innerhalb der Larvenzellen 



