536 G. C. BOURNE. 



ectoderm, and lies in the cavity of the head, separated by a 

 wide space from the head wall, except at its anterior end, where 

 it retains its connection with the ectoderm. In the young 

 larva there is no special nervous apparatus in the oral lobe, but 

 the foundation of the ventral ganglionic chain can be detected 

 in the ectodermic thickenings already described as the ventral 

 plates. At an early period of development special nervous 

 elements are found ventrad of the inner edges of the ventral 

 plates, but lying outside of them ; they are the first foundation 

 of the central nerve-chain of the body. In connection with 

 them rise small bundles of sense-hairs, arranged metamerically. 

 Just beneath these sense-hairs is a mass of large nuclei, derived 

 from the continued multiplication of the ectoderm cells of that 

 region. They lie close upon the endoderm. The inner bound- 

 ary of the ectoderm layer is quite distinct, but at a later 

 period it disappears suddeuly j at this point the endoderm 

 becomes somewhat bent in towards the archenteron, so that a 

 space is formed between it and the ectoderm. Many of the 

 cells, proliferated from the ectoderm in the manner just de- 

 scribed, find their way into this space. One might say that 

 the ectoderm bursts through at this point, emptying its contents 

 as an abscess might, between the previously existing cell 

 layers (see Fig. 1). 



As the result of this process there are established three 

 layers — an ectoderm, a middle layer derived from it, and an 

 endoderm. This division of the ventral plate is only complete 

 in its anterior position, posteriorly it remains undivided. The 

 middle layer gives rise to muscular tissue only, and hence is 

 called muscle-plate ; the external plate, or ectoderm of the 

 ventral plate, may be called the neural plate. It gives rise to 

 the ventral ganglion-chain, sense-organs, and also to non- 

 nervous structures, the sacs of the setae. These seta- sacs are 

 formed by a proliferation of the cells of the anterior parts of 

 the neural plates; they grow inwards as pear-shaped projec- 

 tions, and enter the muscle-plates. In the lateral parts of each 

 neural plate is next developed the dorsal cirrhus as a prolife- 

 ration of cells, and a little later a similar proliferation, close to 



