284 INVEETEBRATA chap. 



(d) The coelom is developed in the same way as in Insecta, and 

 is eventually occluded by the transformation of the cells forming its 

 walls into a fat-body. But before this happens each coelomic sac 

 becomes divided by folds into three portions — a dorsal, a lateral, and 

 a ventral. The first gives rise to the wall of the heart, the pericardial 

 septum, and the genital tubes, as in Peripatus and Insecta. The 

 lateral gives rise to muscles and to certain strings of "lymphoid 

 tissue," and, in the region of the intercalary segment, to two masses 

 lying at the sides of the oesophagus; these are all regarded by 

 Heymons as the degenerate remains of "nephridia" like those of 

 Peripatus. The ventral sections of the coelom, unrepresented in 

 Peripatus, extend towards the middle line, and from their adjacent 

 walls the walls of the ventral vessel are formed. In the genital 

 segment the lumen of the coelom remains undivided and forms the 

 proximal section of the genital duct, leading from the tubular genital 

 organ above towards the genital opening below, as in Peripatus. 



(e) The embryonic area of Scolopendra, marked out by the 

 appendages and underlying somites, grows in length till it extends 

 more than half-way round the egg. Then it is bent into a V shape 

 by a transverse furrow, and infolded into the egg. During this 

 process the two mesodermal bands become widely separated in the 

 middle, meeting one another only at the anterior and posterior ends, 

 and the yolk comes into contact with the much-stretched ventral 

 ectoderm. Eventually, when most of the yolk has been absorbed, 

 the embryonic area is straightened out again. 



By this account we are forcibly reminded both of the early stages 

 in the development of the spider, especially of the process called 

 " reversion," and of the early development of Machilis. But there 

 are certain great differences between Machilis and Scolopendra. In 

 the latter, practically all the much-stretched dorsal ectoderm is 

 eventually incorporated into the adult ectoderm or " hypodermis " ^ 

 but in Machilis a large portion of the dorsal ectoderm undergoes the 

 peculiar histological change described above, and forms " serosa " which 

 is later enfolded into the yolk as the dorsal organ, it degenerates and 

 is digested, and the dorsal "hypodermis" is largely formed by an 

 extension upwards of the ectoderm at the sides of the embryonic 

 area. Nevertheless, in Scolopendra a small portion of the dorsal 

 ectoderm in the neck region thickens and forms a dorsal organ. The 

 cells forming it degenerate and are absorbed, so that in it we have 

 the first rudiment of a " serosa." 



What little is known of the development of Diplopoda or 

 MiUipedes need not detain us long. We owe most of the knowledge 

 we possess to Metschnikoff (1874), and to Heathcote (1886 and 

 1888). It appears that the segmentation of the egg agrees closely 

 with that of Scolopendra. Metschnikoff (1874) describes a division 

 of the surface into radial pyramids, and a conglomeration of nuclei in 

 the central portion. The ovary of Julus is a wide sac beneath the 

 gut, on the walls of which are two bands of germ cells. This sac. 



