298 Tlios. H. Montgomery. 



Introduction and Methods. 



This paper presents an account of the morphological changes dur- 

 ing the ontogeny of Theridium tepidariorum C. Koch, from the time 

 of cleavage to the reversion of the embryo. The considerable gaps 

 in our knowledge of this period of development of the spider and 

 the conflicts of opinion of previous observers, particularly with regard 

 to the formation of the germ layers, seemed to make this study worthy 

 of the undertaking. In the comparisons drawn with the results of 

 others I have limited myself almost entirely to the literature on 

 araneads and have not considered that bearing on other arachnids, 

 for I have had more interest in the ontogenetic processes than in the 

 phylogenetic. Further, I have not treated the literature previous to 

 the classical memoir of Claparede in 1862. Those papers published 

 by Salensky and Morin in Russian I could not read, but have had to 

 rely upon reviews, so that I may not have done full justice to these 

 writers. 



All my embryological material I have secured from spiders kept in 

 captivity at the Woods Hole Marine Biological Laboratory during 

 the summer of 1906, and have been able to get complete and accu- 

 rately timed series of stages with comparative ease. 



All the eggs within a given cocoon are of approximately the same 

 age. The fixing fluid that proved the most satisfactory was that intro- 

 duced by Camoy : glacial acetic acid, absolute alcohol and chloroform 

 in equal parts, with corrosive sublimate to excess. The cocoons were 

 opened, the eggs dropped into this mixture and left in it from one to 

 two hours. By this method nuclear structure and mitotic figures are 

 generally excellently preserved, as well as the cytoplasmic structure. 

 The yolk, on the other hand, becomes generally coagulated, sometimes 

 into a homogeneous mass, also from the stage of about nineteen hours 

 up to about the reversion a yolk extraovat is generally produced in 

 the extraembryonic area. These disadvantages mattered little, how- 

 ever, for I have not given particular attention to the yolk changes. 

 After the preservation *the eggs can be cut with relative ease, for the 

 yolk does not become brittle. 



Stained mounts of whole eggs are most necessary, and beautifully 

 clear preparations were made as follows : After fixation and hardening 



