The Development of Thcridiiim. 327 



the changes in the yolk pyramids than to the cytoplasm and nuclei. 

 Claparede (1862) found the blastoderm to be at first a single layer 

 of cells; and Salensky (1871, Theridium) and Ludwig (1876, Pliilo- 

 dromiis) described the yolk pyramids and movement of the cleavage 

 nuclei to the surface. Then Balbiani (1873, Agelena, Tegenaria, 

 Epeira) studied the cleavage and concluded that the cleavage nuclei 

 move into particular preformed cell territories of the superficial cyto- 

 plasmic layer (couche germinative) ; this particular conclusion of 

 Balbiani's careful memoir has not been substantiated by subsequent 

 students. Balfour (1880) believed each yolk segment to be a cell 

 and the cytoplasm to consist of an envelope for each nucleus and a 

 reticulum around the yolk granules. Sabatier (1881) has added 

 nothing of importance to our knowledge. Locy (1886, Agelena) 

 showed that the supposed cell territories of Balbiani are the result of 

 the pressure of yolk columns upon the superficial blastema, and that 

 these do not come to coincide with the later blastoderm cells; he 

 found also that the cleavage nuclei reach one pole of the egg first, that 

 called by him the "animal-pole," and that all the nuclei are deriva- 

 tives of a single original one. He is the only observer to note a 

 polarity of the egg before cleavage : "One hemisphere is character- 

 ized by small yolk corpuscles packed closely together, though not 

 joined in masses, and the other by agglomerations of larger yolk 

 corpuscles." 



Morin (1887) described a central cytoplasmic mass with fine 

 strands radiating from it, showed that at the eight-cell stage the yolk 

 divides into eight equal masses placed around a cleavage cavity ; then 

 these divide further following division of their nuclei, forming 

 rosettes, and ultimately all the cells reach the surface of the egg. 

 Schimkewitsch (1887) corroborated in some points Ludwig and Locy, 

 found that the number of yolk pyramids at the end of the cleavage 

 varies with the species; there is no independent superficial layer of 

 cytoplasm, but the protoplasm occupies the center of the egg; later 

 (1898) he showed, correcting his conclusions of 1887, that all the 

 cleavage nuclei reach the surface to form the blastoderm, none 

 remaining within the yolk. Kishinouye (1890) calls the central 

 cytoplasm the "centroplasm," and the peripheral, the "periplasm," 



