THE LUNG-SACS — THE INTESTINAL CANAL. 19 



■describes three pairs of depressions, one in each segment ; the one in the middle 

 must be identical with the cephalic pits of other authors, its transformation has 

 already been described (pp. 12-15). These middle pits give rise simultaneous/// 

 to the median eyes and the optic ganglia, the latter sinking in with them 

 (Fig. 12 E). The formation of the optic nerve is thus eccsy to explain (Fig. 12 

 ■C and D). The optic ganglia are connected with the cranial part, which has 

 meanwhile also shifted inwards. 



The first segment has no eyes, but the third carries the lateral eyes ; these, 

 however, are not invaginated. "We cannot here follow Patten's account further, 

 as his conclusions do not seem sufficiently supported, and his statements are too 

 fragmentary. It is also impossible to decide as to the value of Patten's 

 statements, which, as the title of his treatise (No. 27) shows, were made with 

 another object ; indeed, his method of description often makes it impossible for 

 us to obtain even a slight idea of the formative processes observed by him. 



B. The Lung-sacs. 



The lung-sacs arise, as has already been mentioned (p. 11), as 

 depressions on the posterior sides of the last four abdominal limbs 

 {Metschxikoff, Laurie, Fig. 8, p. 11). These depressions are at 

 first shallow, but then grow deeper and spread forwards, extending 

 in front of their narrow apertures, which correspond to the future 

 stigmata (Fig. 9 C, st). The sacs project into a vascular mesodermal 

 cavity (Kowalevsky and Schulgin, Laurie). 



The assumption of the adult form by the lung-sacs takes place in 

 the latest embryonic stages ; it commences by the occurrence of 

 depressions in their inner walls. These lead to the formation of 

 folds which grow out further and further into the cavity of the sac 

 (i.e. posteriorly). Other folds form, and the lamellate structure of 

 the sacs thus gradually arises (Laurie). The wall of the embryonic 

 lung-sac consists of a cylindrical epithelium (hypodermis), which 

 secretes a fine cuticle on the surface turned towards the cavity of 

 the sac (Metschnikoff).* 



"We shall deal further with the development of the lung-sacs and their 

 morphological relations when we come to treat of the respiratory organs of the 

 Araneae (p. 76). 



C. The Intestinal Canal. 



The enteron develops somewhat differently in the different regions 



of the body. While the entoderm is represented by a single layer of 



cells wherever it surrounds the yolk, in the post -abdomen it first 



appears as a solid cell-mass (Fig. 13). In the post-abdomen, the 



* [Laurie (App. to Lit. on Scorpiones, N"o. IT.) states that the lamellae of 

 the lung-book in the embryo lie horizontally and parallel to the ventral surface, 

 while in the adult the lamellae are arranged parallel with the axis of the body 

 and perpendicular to the ventral surface. He supports MacLeod's rather than 

 Laskestek's view concerning their origin. — Ed.] 



