vi HEXAPODA EMBRYONIC DEVELOPMENT 497 



become connected with the mid-gut at a later stage. The Malpighian vessels arise 

 as imaginations from the ectodermal proctodseum. 



The limbs (Fig. 353) appear as paired bud-like outgrowths of the ectoderm and 

 the subjacent mesoderm ; they appear between the primitive thickenings of the nerve 

 chord and the stigmata. 



It is an important fact that besides the rudiments of the head and thoracic limbs, 

 which alone are present in the adult animals, there occur at a certain stage distinct 

 rudiments of abdominal limbs as well, those of the first abdominal segment being 

 particularly clear (cp. especially Fig. 353, J5). These rudiments degenerate later. 



The origin of the two endodermal cell layers, which we saw appear in the 

 transverse section (Fig. 350, F), is difficult to explain. At the stage at which this 

 section is taken, the whole endodermal rudiment consists of two lateral cell streaks 



A ^ 



I % 



*Jt 



p* / 



af 



Fig. 352. A-E, Ventral view of 5 stages in the development of Hydrophilus (after Heider). 

 The anterior end is directed upward, a and b, Points at which the blastopore first closes ; af, edge of 

 the amnion fold ; af, caudal fold ; af", paired cephalic fold ; an, antenna ; es, terminal segment ; 

 g, pit-like invagination (rudiment of the amnion cavity) ; fc, procephalic lobes ; r, groove-like 

 medio-ventral invagination ; s, germ-streak covered by the amnion. 



between the yolk and the two rows of mesoderm cavities. Anteriorly at the 

 stomodseum and posteriorly at the proctodseum the two streaks pass into one 

 another. At earlier stages the endoderm consists of an anterior and a posterior 

 U-shaped double streak. The limbs of the U, which are very short to begin with, 

 are at first directed backward in the anterior double streak, and forward in the 

 posterior streak ; they gradually elongate till at last the two anterior limbs meet the 

 two posterior, and thus give rise to the two above-mentioned endodermal streaks. 

 The first origin of the two U-shaped double streaks must probably be sought in the 

 anterior and posterior ends of the median invagination of the blastoderm of the 

 embryonic rudiments, which elsewhere forms only mesoderm. The whole invagina- 

 tion would thus be the rudiment of the mesoderm plus the endoderm, and might 

 be regarded as the gastrula invagination. In a transverse section of the still open 

 anterior part of the invagination its base would represent the formative zone of the 

 U-shaped endodermal streaks, while the lateral somewhat bulged-out walls yield 

 mesoderm. The mesodermal germ streak would thus be continued anteriorly in two 

 lateral bulgings of a gastrula invagination, arid we are therefore inclined to regard 

 the mesodermal formation of insects as a modification of that manner of forming the 

 mesoderm in which it proceeds from paired invaginations of the archenteron. 



According to another view, the yolk with its nuclei represents the endoderm, and 

 VOL. I 2 K 



