The Development, of Theridiiim. 339 



by Salensky to compose a vascular sinus, while Balfour, Locy, 

 Morin, Schimkewitsch, Jaworowsky found they simply disappear, 

 and Korschelt suggested they might form the tracheae. The 

 appendages of the fourth and fifth segments become the spinnerets, 

 as found by Salensky and con-oborated by Locy, Morin, Kishinouye, 

 Wallstabe and Jaworo-svsky (1895) ; the last author has given the most 

 detailed account of them, concluding the appendages of the fourth 

 segment form the anterior spinnerets and the cribellum (or colulus), 

 and those of the fifth segment the median and posterior spinnerets. 



15. Growth Differences of Cephalothorax and Abdomen. 

 The cephalothorax and abdomen show certain striking differences 

 in their growth and diiferentiation. The boundary between the two 

 can be recognized as a thin region of the ectoblast, a little anterior to 

 the posterior cumulus at an early stage before there is any trace of 

 segmentation, the posterior cumulus practically coinciding with the 

 position of the caudal lobe. Thus, nearly the whole extent of the 

 cephalothorax is laid down, thereafter to lengthen but little more, 

 when the abdomen is represented only by its future posterior end. 

 Then with the segmentation of the mesoblast the abdomen com- 

 mences to increase rapidly in length, pushing around upon the yolk 

 until the caudal lobe finally comes into contact with the head lobe. 

 The abdomen is in most of its extent later in origin than the cephalo- 

 thorax, and its growth is strictly teloblastic by the formation of 

 successive segments from its anterior end caudad ; its rapid increase 

 in length comes suddenly at the close of the gastrulation period. 

 This rapid elongation of the abdomen explains why the caudal lobe 

 is narrower than the head lobe. Segmentation appears first in the 

 cephalothorax, and its five posterior segments are well marked 

 before the first abdominal becomes evident ; further, these five ce- 

 phalothoracic segments seen to appear simultaneously or nearly so 

 and not in sequence from before backward, while the cheliceral seg- 

 ment, the most anterior, is the one to form latest. Therefore the 

 segmentation of the embryo as a whole is not teloblastic, but only 

 that of the abdomen; regular teloblastic growth takes place only 

 between cephalothorax and the caudal lobe. Another and perhaps 



