HABITS, ETC., OF CEREBRATULUS LACTEUS. 173 



But not all tlie mesencliyme is employed in the manu- 

 facture of muscle. Certain of the macromesencytes do not 

 develop processes or exhibit amoeboid movements, but divide 

 and subdivide actively. As fast as they are used up in this 

 way others take their place, so that they may always be found 

 scattered about in the gelatine. In the living larva they 

 appear yellowish, and they always stain more deeply than 

 the muscle cells. Evidently they are the source of the gela- 

 tine itself, and probably correspond to the isolated cells which 

 Biitschli showed (11) were transferred to the adult Nemer- 

 tean. If their development could be followed in this species 

 it would doubtless be found that they share the same fate. 



Summary. — 1. The mesenchyme originates from large 

 entoderm cells situated at the junction of entoderm and 

 ectoderm. 



2. Its cells are of two kinds, called macromesencytes and 

 micromesencytes respectively, the former being several times 

 larger than the latter. At first macromesencytes are given 

 off alone, then both kinds appear together. From micro- 

 mesencytes are developed the parietal and lappet muscles ; 

 from macromesencytes all the other muscles of the pilidium. 



3. In this muscle development both kinds of cells may 

 either remain single, forming delicate fibres, or may anasto- 

 mose into a complex network, or several cells may fuse into a 

 larger strand. 



4. At first the macromesencytes float about freely and 

 exhibit slight amoeboid changes of form. Then they put 

 out slender pseudopodia branches which arrest their motion. 

 These pseudopodia do not show any contractile movements, 

 but only a streaming motion of granules and cytoplasm. 

 They are lengthened until they come in contact with some- 

 thing to which they can be fastened. 



5. As soon as they are fastened the structure changes from 

 that of normal cytoplasm into typical fibrillated muscle. 

 This is accompanied by a corresponding change in activity ; 

 the streaming ceases, and the fibrils become contractile and 

 function as muscles. 



