HISTOLOGICAL DIFFERENTIATION OF THE EMBRYO. 



341 



-\/ft , 





FtG. 182. — A piece of a rabbit's liver with 

 bladder- worm passages (Oysticercus pisiformis) 

 ( X 10). 



smaller, and measures not more than about 01 mm. Embryonic 



hooks cannot, therefore, be demonstrated with any certainty. A 



histological differentiation now 



sets in, and is inaugurated by a 



clearing up of the central portion 



of the body-mass. This is due 



to large, clear, non- granular 



vesicles, which have an almost 



drop-hke appearance, and which 



grow so rapidly that the inner 



parenchyma is soon seen as a 



distinct tissue, different from the 



cortical layer. In the second 



week one can distinguish in the 



worm, which has now grown to 0-5 mm. or more, the first traces of 



the subsequent musculature through the not inconsiderably thickened 



cuticle. These are in the form of fine fibres, which run in close 



groups in two directions at right angles to one another; the outer 



are the circular, the inner the longitudinal fibres. In appearance and. 



arrangement they resemble the subcuticular musculature of the adult 



Cestodes (p. 290). 



One must not suppose that the worms _ 



could not move before these muscular fibres 

 were distinguishable ; in fact, the form of the 

 body is seen to change, constricting itself here 

 and there, the contractions passing peristalti- 

 caUy up or down. The motions are, indeed, 

 weak and slow, but even the musculature is 

 not able at first greatly to accelerate them. 



The cortical layer, running under the sub- 

 cuticular muscles, consists of small, closely 

 pressed nucleated cells, with numerous shining, 

 fat-like molecular bodies often grouped to- 

 gether. Next there are the above-mentioned 

 drop-like vesicles, which have between them a 

 clear, sparsely developed substance, and are 

 considerably larger than in the former stage. 



But after a short time the cortical layer undergoes a further 

 differentiation. The cells, of the deeper layer at least, form a clear, 

 tough, connective tissue mass, penetrated by numerous cross muscular 



variations, as may be strikingly seen by comparing the young bladder-worms of the 

 brain (Cysticercus cellulosw) with those found in the muscles, but even in the same organ 

 we usuaUy find bladder-wor^Q|^d^^ey|jSizj|^^^^j^^iation. 



Fig. 183. — Cysticercus pisi- 

 formis before the deve- 

 lopment of the head, with 

 granular sheath and cyst 

 (X60). 



