MYRIAPODA 389 



The preantennal and the antennal segments each have a pair of append- 

 ages, although the former with its appendages is evanescent, present 

 only in the embryo. The clypeus and lab rum soon occupy a ventral 

 position; the dorsal part of the acron together with that part of the 

 membrana dorsalis which belongs to the gnathal or jaw segments fuse into 

 a plate known as the lamina cephalica. The remaining postoral segments 

 of the head are the intercalary, the mandibular, and the first and the 

 second maxillary segments, each with distinct coelomic sacs and, with 

 the exception of the intercalary segment, with appendages also. The 

 intercalary segment is apparent in the embryo only. The hypopharynx 

 is developed from the sternite of the mandibular segment alone, 

 the sternite of the first maxillary playing no part in its formation. The 

 appendages that will develop on the first segment behind the head are the 

 prehensile maxillipeds joined to a broad sternocoxal plate. Following 

 the maxilliped segment are 21 rump segments, each bearing a pair of leg 

 rudiments. The telson, bearing the anus, first develops in the form of a 

 horseshoe but later becomes transversely oval. The coelomic sacs are 

 lacking in it. Between it and the last leg-bearing segment is an inter- 

 polated segment which later divides transversely, forming the twenty- 

 second and twenty-third rump or the twenty-ninth and thirtieth postoral 

 segments which Heymons calls the "pregenital" and "genital" segments, 

 respectively. Though small, they are typical metameres, each with a 

 pair of coelomic sacs. 



Derivatives of the Mesoderm. — The early differentiation and seg- 

 mentation of the mesoderm have already been mentioned. The meso- 

 derm lies in the form of two lateral bands with segmental swellings, inside 

 which (toward the yolk) is the entoderm in the form of a sheet of flattened 

 cells (Fig. SAOA,ent), which hes not only in the region of the germ band 

 but, in contrast to the mesoderm (mes), extends between blastoderm (Fig. 

 3^0B,dm) and yolk. Soon the greater number of mesoderm cells (mes) 

 take on a cubical cylindrical form and attach themselves to the ectoderm ; 

 others, more flattened, lie near the entoderm, the former later forming the 

 somatic, the latter the visceral wall. At first closely in contact with each 

 other, these mesoderm cells later separate, leaving a cleft between them, 

 the coelomic cavity. Six pairs of these sacs are developed in the head, a 

 pair in the maxilliped segment, and a pair in each rump segment; only 

 acron and telson lack them. At first the sacs are lenticular; but as soon 

 as the buds of the appendages (p) appear, the somatic layer follows the 

 evagination whereby the lumen of the sac increases in size (Fig. 346, coel), 

 that of the antennae being largest. The coelomic sac of the preantennal 

 segment likewise is large and sends forward a flat process into the preoral 

 region (Fig. 347, pren). The coelomic cavity of the intercalary segment 

 (int) is small and evanescent. Mesoderm not used in the formation of 



