244 Post-embryonic Development of Julus terrestris. [Dec. 8, 



These cells which were directly derived from the hypoblast in the 

 early stages of development, form a network in the body-cavity. The 

 heart is the result of a joining together of the meshes of this network, 

 and thus is formed by the confluence of a series of spaces in the 

 mesoblast, and has nothing to do with the development of the ccelom. 

 The heart is placed in the middle dorsal line between the gut and the 

 body- wall. It has two pairs of arteries leading into the spaces of the 

 fat body in each double segment, and two pairs of ostia. The part of 

 the body- cavity in which it lies is shut off from the rest of the body- 

 cavity by an imperfect pericardial membrane which is continuous 

 with the fat bodies. The tube of the heart is composed of three 

 coats, an inner structureless membrane, a median muscular coat, the 

 fibres of which are disposed circularly in alternate broad and narrow 

 bands, and an outer connective tissue coat. The fat bodies are also 

 formed from the same network of mesoblast cells which in this case 

 secrete oil globules. 



The body- cavity is a series of spaces between the gut and the body- 

 wall, and is divided up by the mesoblast cells already referred to. It 

 is distinct from the ccelomic cavities of the somites, and is therefore a 

 pseudoccele. 



The eye-spots are all formed in the same manner. The hypo- 

 dermis thickens and a cavity appears within it bounded by pigment. 

 This cavity becomes a distinct vesicle. The front wall of the vesicle 

 becomes very thin and furnishes the lens, while the cells of the back 

 {i.e., most internal) wall and sides become elongated and form the 

 retinal elements of the eye. The nuclei of the front wall become 

 very faint and finally disappear, while the rest of the vesicle remains 

 continuous with the hypodermis of the body-wall. The cells of the 

 vesicle are at first separate from the ganglion cells of the nerve- 

 system, but a connexion takes place very early. A number of very 

 small cells appear within the walls of the vesicle at a very early period, 

 and I believe them to be derived from the mesoblast cells in the body- 

 cavity, but of this I am not certain. They eventually become the 

 pigment cells described by Grrenacher. 



The most striking feature of the development is the reduction of 

 the ventral part of the young animal and the increase of the dorsal. 

 In the just hatched animal the ventral region is nearly as large as the 

 dorsal, and the legs are wide apart, having a distinct space between 

 them. As development progresses the dorsal region is increased, 

 while the ventral is contracted till the bases of the legs are close 

 together. The corresponding concentration of the nerve-cord I have 

 already mentioned. In a paper on Euphoberia, a Carboniferous 

 Myriapod, Mr. Scudder points out that one of the principal points in 

 which the genus differs from existing Diplopoda is the development 

 of the ventral region. The relations of the dorsal and ventral regions 



