232 



INVEETEBEATA 



CHAP. 



the site of the central eyes. The lateral eyes are formed as 

 depressed circular areas in the ectoderin, one on each side of the 

 head, i.e. at the edges of the ventral plate. The lateral vesicles are 

 now completely cut off from the exterior, and Kishinouye compares 

 them to the eyes of Peripatus. 



The chelicerae have been shifted forward so that their bases 

 nearly meet in the mid- ventral line in front of the mouth, and their 

 ganglia form the commissure which connects the brain with the large 

 suboesophageal ganglion. The latter is formed from the ganglia 

 of the pedipalps and the four ambulatory limbs ; these ganglia are in 

 close contact with one another, but at this stage have not yet fused. 



The above concludes the account of the changes which take place 

 during the process of reversion. When this process is complete the 

 undigested yolk forms a huge semicircular protrusion on the ventral 



Fig. 182. — Surface views of the cut-off alDdomen of three embryos of Agdena labyrinthica 

 of different ages in order to show the modifications undergone by the abdominal 

 appendages. (After Kautsch.) 



aV^-ab^^ tlie abdominal appendages ; aW> (on the right side of Fig. C), the inner part of the last 

 abdominal appendage, whicli gives rise to the tliird spinneret ; c.l, caudal lobe ; V^, the last leg ; prod, 

 proctodaeal invagination : tr, invagination to form the trachea. 



surface, and the mesodermic bands are situated somewhere about the 

 equator of the egg, which still retains its spherical form. 



In the succeeding period of development, which lasts until birth, 

 the yolk becomes absorbed, and as it disappears the ventral 

 ectoderm contracts in width, and the two halves of the nerve cord 

 and the two mesodermic bands approach one another once more. At 

 the same time the ventral ectoderm grows in length and forms a deep 

 inwardly directed fold which is the beginning of the constriction 

 separating abdomen and prosoma from one another. During this 

 period the various organs complete their development. 



We may commence by the consideration of the central nervous 

 system and eyes. The grooves in the brain become now completely 

 closed off from the exterior and form crescentic tubes. The inner 

 portions of these crescentic tubes completely fuse with one another 

 so as to form the stem of a T, the transverse arms of which are 



