406 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



inents of the appendages. The third period may be divided into three 

 parts: (1) the truly embryonic, until the first ecdysis and the rupture of 

 the chorion; (2) the first fetal or first postembryonic stage, ending in the 

 second ecdysis; and (3) the second fetal or transition stage, when the 

 body elongates and possesses spontaneous movement. In the fourth, 

 or adolescent, period there are several molts, ending with the maturing of 

 the sex organs. 



In essential features the structure and development of the heart of 

 LithoUus forficatus as described by Biegel (1922) resemble that of 



Scolopendra. The aorta, however, is 



^^■^^ ^'"■■■\ not differentiated, and the aortic 



/ ,...-^.^ .'---v, \ arches are lacking. In place of them 



/ iJ^^Alf^) \ 1 a large head blood sinus functions. 



Lateral transverse vessels likewise are 

 replaced by a segmentally arranged 

 system of blood lacunae. Above the 

 heart is a small dorsal heart sinus. 

 Instead of the asymmetrically ending 

 genital duct, in Lithobius forficatus 

 both right and left ducts are functional. 



DIPLOPODA, THE MILLEPEDES 



Fig. 362. — Platyrhacus amauros. 



Platyrhacus amauros Attems 



This species is found in the decay- 

 Germ disk before Aexure (a) Anus . ^^^ ^^ ^^^ ^^^^ j^^ ^^^ ^^^^^ 

 {ant) Antenna, {ma) Mandible, {mx) o • i /. . i • 

 Maxilla, {prz) Proliferation zone, plants On the island of AmboiUa 



{From Pflugf elder.) (Moluccas). The cggs, which are laid 



in loose clumps of 60 to 80 in mulch, measure about 1 mm. in diameter, 

 are rich in yolk, and hatch in about 19 days. According to Pflugfelder 

 (1932), the cleavage nuclei surrounded by a thin layer of cytoplasm and 

 more or less connected with each other by cytoplasmic strands migrate 

 to the periphery where they undergo further division to form a blasto- 

 derm of flattened cells. A few yolk cells remain behind in the yolk. 

 More active division of the cells on the future ventral half of the egg 

 causes crowding and consequent deepening of the individual cells. 

 The area of deep cells is elliptical in outline and constitutes the germ 

 band, or disk. 



A primary division of the disk gives rise to a head and a tail region 

 between which there is an unsegmented middle section. Shortly follow- 

 ing this, further segmentation, first of the head and thorax and finally 

 of the abdomen, takes place. The germ disk (Fig. 362) now consists of 

 protocerebral and deutocerebral segments still united on the lateral 



