330 



EMBRYOLOGY 



of cells, extending over from one to the other (Fig. 156). 

 From these head germs the whole head portion is said to be 

 formed, including even the epidermis, for the epidermis 

 which is now present (Fig. 156 ep) is of only a provisional 

 nature. In like manner the entire trunk portion (with the 

 exception of the mid-gut) is said to arise from the trunk 

 germs. The head germs and trunk germs unite in the region 

 of the mouth. Thus in the formation of the body a decided 

 difference would exist between head and trunk. 



Fig. 156. — Longitudinal section of a larva of Nephelis (after Bergh). ent, ento- 

 dermal elements; ep, provisional epidermis; kk, head germ {Kopfkeim) ; m, 

 mouth-opening ; mex, individual mesoderm cells; mu, musde cells; pz, pole cells 

 of the germ band (i.e. trunk germ) ; rfc, trunk germ {Rumpfkeim) ; s, provisional 

 pharynx; urii and uu„ primitive kidneys or their fundaments. 



Whitman also assumes a fundamental distinction between head and 

 trunk portions, and is inclined to refer the origin of the former to the 

 four micromeres first formed. However, the difference in the Clepsine 

 observed by Whitman is not so striking as here, for in that case the 

 epidermis is not cast off. 



If the permanent body of the medicinal leech is really formed from 

 four fundaments, then the comparison with the formation of the 

 Nemertean from the Pilidium, which was attempted by Bebgh, is a natural 

 one. In Pilidium also the larval skin is cast off, and the body arises 

 from several separate fundaments, of which the mesodermal are four in 

 number (two in the head and two in the trunk portion) (comp. p. 223). 

 Yet these processes, as far as they are known, appear to take place in 



