The Development of Theridium. 299 



the chorion was removed with needles, the egg then overstained in 

 Delafield's haematoxyline. Destaining was then carried out in acid 

 alcohol until the egg became a pale pink color, with all stain removed 

 from the yolk and cytoplasm ; xylol was used for clearing and Canada 

 balsam for mounting. When the ectoblast has secreted a cuticula, as 

 at the time of reversion, it is necessary either to stain longer or else 

 to remove a portion of the embryo with a scalpel, so as to allow satis- 

 factory penetration of the stain. For sectioning the chorion was 

 removed, the embryo stained in eosin in the absolute alcohol used for 

 dehydration (so as to make it clearly discernible in the paraffine), and 

 cleared in xylol. Serial sections were cut 7 micra thick, and these 

 stained with either Delafield's haemotoxyline or iron haemotoxyline 

 followed by eosin. The embryos were so imbedded in rows that sev- 

 eral could be cut at once, whereby sections in all desirable planes 

 could be quickly secured. I have mentioned these methods somewhat 

 in detail, for eggs of spiders have generally been considered difficult 

 to treat. 



Theridium tepidariorum is an exceptionally favorable form for 

 study, because timed stages are secured with facility, the females 

 being easily kept, and the eggs are conveniently small. 



1. Observations. 

 1. The Cleavage Up to Gastrulation. 



The first cleavage spindle is found at from 3 to 4 hours after ovi- 

 position; the two-cell stage at from 4 to 5^ hours, and the four-cell 

 stage at from 6 to 7 hours. 



In PI. I, Fig. 1, is seen the anaphase of the first cleavage, the com- 

 mencement of the two-cell stage. The daughter nuclei lie each 

 adcentral within a mass of cytoplasm, and these central cytoplasmic 

 masses are connected with the thin peripheral layer (blastema) by a 

 network of delicate strands coursing between the yolk columns. In 

 the figure indicated only a few of the coarser of these strands are 

 shown, for most of them are too delicate to be shown at this scale of 

 magnification. My account of the fertilization (1907) explained how 

 this protoplasmic arrangement comes about; how the young oocyte 

 contains dense cytoplasm from the center to the periphery, and how 



