- JOHN W. MACARTHUR. 



Herbst demonstrated quite conclusively ('95 a, e.g. Figs. 17, 

 18, 22) that the thick entodermal layer of the lithium larvae and 

 of the intermediate types, is rich in nuclei and in the number of 

 its smaller cells in proportion to the size of the part. The ecto- 

 derm, on the other hand, gives every evidence of retarded ac- 

 tivity: at first very thick, it afterward becomes thin and stretched 

 but is never composed of more than a single cell layer (except 

 when and where the ciliary bands appear), and this layer is 

 peculiarly sparse in nuclei and is composed of a few extraordinarily 

 large cells, both in comparison with the normal ectoderm and 

 with the entoderm of lithium larvae. 



The tip of the fore-gut often shows some disintegration, and 

 the resulting cellular debris and associated mesenchymal cells 

 are evidently somewhat sticky, for one finds frequently that 

 two exogastrulae fuse, telosynaptically as it were, by the ends 

 of their guts (not laterally as in Herbst's cases). These adhere 

 closely and more or less permanently to form " fused twins" 

 (Fig. 3, w, x) whose parts, of varying relative size, have an in- 

 teresting history of inhibition and reciprocal overgrowth, separa- 

 tion, etc., that might be followed further with profit. 



Meanwhile differentiation has progressed somewhat in the 

 ectodermal component also; especially if this is fairly large, has 

 not been too strongly inhibited, and is then allowed to recover 

 partially in sea water. Notable deviations from the normal 

 development are seen in the marked reduction and generally 

 radial disposition of apical structures. The mouth usually 

 fails to form, but when it does it is practically terminal. The 

 ciliary bands, when formed, appear as an apical ciliated tuft, 

 plate, or small transverse ring. Apical outgrowths are common 

 (Fig. 3, d). By the third day skeletal spicules have often formed 

 in the mesenchyme, which is collected, not in two lateral masses 

 near the blastopore, but, rather, crowded far forward into the 

 apical end of the ccelome; these spicules stimulate the produc- 

 tion of the pluteus arms, \\hich are uncommonly close together 

 and narrow angled from the marked reduction of the oral lobe 

 and also from the tendency of the "anal" arms to be formed 

 very near the oral region. Herbst also noted ('95, a, Figs. 32-36) 

 that lithium larva? successfully recovering in sea water continue 

 their overproduction of mesenchyme and skeletal structures and 



