THE BIOLOGY OF THE CELL SURFACE 



can easily confirm under low power of an ordinary 

 microscope. Indeed, I have with the naked eye seen the 

 green cytoplasm on this rather large egg; also have I 

 found that under pressure the endoplasm, which Kowalew- 

 sky did not consider living substance, will stream out as 

 the large q^^ ruptures leaving the green surface-cytoplasm 

 intact. 



The diflFerence between the peripheral and the inner cyto- 

 plasm of eggs of flat-worms has been often observed,^ 

 though some authors fail to do so, and still others have 

 expressed the opinion that the structural differentiation 

 observed might be due to fixation.^ Many flat- worm eggs 

 present serious difficulties for study in the living state 

 because they are laid in capsules each containing several 

 eggs; removal of the eggs from the capsule may mean their 

 injury. In addition, because of opacity, they do not always 

 lend themselves to observation of their finer structure. 

 Nevertheless, one may assert that eggs of turbellarians and 

 trematodes and even of those of cestodes (tape-worms) 

 exhibit a resolution of the cytoplasm into an endoplasmic 

 and an ectoplasmic region, as Metschnikoff, Selenka, Lang, 

 Pereyaslowzewa and Janicki, among others, have shown. 



The thread-worms, nematodes, include Ascaris, a para- 

 sitic worm which has been the object of many researches 

 and which will come up for reference frequently in this 

 book. The unfertilized egg of Ascaris shows, accord- 

 ing to Van Beneden and Meves, ectoplasmic differen- 

 tiation; irregular in shape, after fertilization it becomes 

 spherical having in the meantime secreted a substance 

 which forms an enclosing shell. The eggs of free-living 

 nematodes, closely allied to Ascaris, show a similar change; 



^ Metschnikoff, /SSj; Selenka, i88j; Lang, iSS^; Pereyaslowzewa, 

 iSg2; Warren, igoj; Janicki, igoj. 

 - Halkin, /go2. 



92 



