140 EXTERNAL FACTORS III. 8 



region can, however, only be realized when the salt is allowed 

 to act at a stage in the blastula when the differentiation into 

 the two primary layers is already beginning. Should the 

 embryos be removed before this stage is reached, after twenty- 

 four hours' exposure to the solution, only ' Exogastrulae ', not 

 1 Holoentoblastia/ can be obtained. Should, on the other hand, 

 older blastulae, or gastrulae, or Plutei be placed in the solutions, 

 they die without showing any signs of the characteristic abnormal 

 development. From the fact that equimolecular solutions of 

 monobasic lithium salts produced like effects (such solutions, it 

 must be observed, are also chemically equivalent), Herbst con- 

 cluded at first that the osmotic pressure was responsible for the 

 abnormalities ; but the permanent after-effects of temporary im- 

 mersion just referred to subsequently convinced him that the ova 

 were permeable to the lithium ions to which he now attributes 

 the specific nature of the monstrosity. He suggests further that 

 they act upon the endoderm cells by increasing their absorptive 

 activity and their power of cell-division, while at the same time 

 they inhibit the functions of those mesenchyme cells which are 

 devoted to the formation of the skeleton. 



As in other monstrosities, there is an alteration in the pro- 

 spective potentialities of cells, elements which would normally be 

 ectodermal becoming converted into endoderm, and additional 

 mesenchyme cells being involved in the secretion of skeletal 

 spicules. 



It is only by lithium salts that the typical ' Holoentoblastia ' 

 can be produced; but Exogastrulae can be reared in others, in 

 sodium butyrate, for example ; in this solution a stomodaeum is 

 formed, but is, like the archenteron, everted. Even lithium, 

 however, is powerless to cause the ' holoentoblastic ' reduction of 

 the gastrula wall in the larvae of Asterias, although exogastru- 

 lation may, but need not, occur. A characteristic deformity is the 

 absence of the pre-oral region, and the elevation of the mouth 

 on a sort of hypostome. In Ampfiioxus and Ascidians it is 

 impossible to obtain even exogastrulae by these methods. It 

 is evident, therefore, that the specific morphological reaction 

 depends not only on the nature of the substance employed, but 

 also on the constitution of the reacting organism. 



