288 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



and joined together at short distances, so as to have a ladder-Uke or lattice structure. 

 As the skeleton appears early in the larvte, it is obviously easy to investigate the 

 inheritance of these differences in the young hybrids, without the necessity of being 

 forced to raise them to a late stage, the skeleton being completely formed by the end 

 of the seventh or eighth day. Thus we have a large amount of evidence with regard 

 to the inheritance of the skeleton, and this is chiefly remarkable for its very conflicting- 

 nature ; frequently investigators working on the same material in the same laboratory 

 have obtained diametrically opposite results — as for instance Boveri (9) and 

 Morgan (65) at Naples, and Loeb(54) and Haoedoorn (38) working at Pacific Grove, 

 California. 



A study of the development of the skeleton in pure-bred larvfe shows that it is 

 highly influenced in its growth by any slight unhealthiness of the larvae, changes in 

 the sea-water, or small metabolic disturbances taking place within the larvae, through 

 improper or excessive abundance of food. Thus the skeleton under perfectly normal 

 circumstances varies within wide limits. The extensive papers of Vernon (94), 

 DoNOASTER (22), and Tennent (89) have drawn attention to this great variation. 



Moreover, the majority of workers in this field have paid little attention to rearing 

 their larvae, usually placing them in small jars of sea- water, and allowing them to 

 develop as far as they will go without food. The food-stuff stored in the egg carries 

 development a certain way without any undue disturbance of the normal course of 

 events. After this, however, the larvae require a constant supply of food, or their 

 growth remains stationary, and rapidly becomes abnormal. The eighth day of 

 development corresponds roughly with the appearance of the postero-dorsal arms, 

 shortly after which the skeleton reaches its maximum period of formation. In the 

 normal course of affairs, it rapidly undergoes reduction after this date. If the larvae 

 i-emain without food their growth remains stationary, and they form excess of skeleton 

 substance, until they become filled with calcareous plates and rods of every size and 

 structure. It seems, when the development of the pluteus is abnormal (for example, 

 if the limbs are stunted, or one or more are absent, or if development is retarded and 

 the animal remains dwarfed), that the skeleton continues to grow, but in a highly 

 irregular way, branching out and invading regions of the pluteus where it would not 

 under normal circumstances appear. It is as if there were a certain amount of skeleton- 

 forming matter to be disposed of, and that this must become skeleton, although the 

 pluteus is retarded in growth, and the skeleton support is not required. The skeleton 

 matter thus proliferates into complex arrangements, spines, and rods. This frequently 

 takes the form of an additional rod, in one or both of the postoral arms ; while under 

 normal conditions of development, there is one rod. In E. esculentus, for instance, 

 there is more or less straight-rod form of skeleton in the postoral arms, never imder 

 perfectly normal conditions approaching the ladder or lattice form so characteristic of 

 SphcBrechinus granularis. Yet the pure-bred E. esculentus pluteus, when it reaches 

 the eighth day, very frequently develops multiple rods or even an irregular lattice 



