ON THE DEVELOPMENT OF ARANEINA. 69 



anal lobe and of the four appendage-bearing abdominal segments 

 have attained their utmost development. The first abdominal seg- 

 ment and those between the fifth and the last abdominal segments 

 are aborted. 



The mesodermic somites which are produced at first in the 

 ventral plate now grow on dorsalwards and meet at the dorsal 

 median line (PL XV, figs. 40-43). They first meet at their dorsal 

 part, enclosing some of the large fat cells and their derivatives be- 

 tween them. The ventral part fuses later. Thus the dorsal circulatory 

 tube is formed, the wall of which is produced from the mesoderm, 

 while the blood corpuscles are produced from large fat cells (endo- 

 dermic in origin). I am inclined to believe that both the aorta and 

 the so-called heart are formed as stated above and not separately as 

 many authors believe. The fusion of the mesodermic somites to 

 form the dorsal vessel does not take place throughout the entire 

 length, as there are left paired lateral slits between each two consecu- 

 tive somites. The blood aerated at the lung-book returns to the 

 heart through these lateral slits. These slits shut and open as the 

 heart beats. They are found in the abdomen only. 



In the basal part of the first abdominal appendage of each side, 

 there arises an ectodermic invagination whose opening faces away 

 from the median line. It is neither deep nor spacious but is a little 

 pocket-like invagination. This is the beginning of the lung-book. 

 The development of this organ, briefly stated, is as follows : Of the 

 wall of the invaginated pocket, that which faces the distal end of the 

 appendage is much thicker than the opposite wall, filling the interior 

 of the appendage. The cells composing it become after a while 

 arranged in parallel rows (figs. 34 and 47). Each two of these 

 parallel rows adhering together produce the lamellae of the lung- 

 book. The external epithelium of the appendage which cover these 



