254 MYRIOPODA. 



secondary. Further, it is of interest in this connection that the 

 eggs of the lowest Insects (Podura) are also said to undergo total 

 cleavage, although we are still without certainty on this point. 



The formation of the germ-layers in the Myriopoda is still too 

 little understood for us to draw definite conclusions from it, but 

 the external form of the body offers a few points of comparison, 

 though these are perhaps not very certain. It was shown that the 

 Myriopodan egg undergoes a decided ventral folding at an early stage 

 of development, and that this may lead to the sinking of the whole 

 germ-band into the yolk (Figs. 113-116, pp. 226-230). This also 

 appears to be the case in Peripatus, judging from the statements 

 and drawings of L. Sheldon, and it seems not impossible that these 

 developmental processes which occur in the Myriopoda and the 

 Insecta are foreshadowed in Peripatus.* 



Indications of a low grade of development, and at the same time 

 of resemblance to Peripatus, are afforded by the suggestion of 

 ventral organs (of head and trunk), the continuation of the primitive 

 segments into the extremities, especially into the antennae, the 

 condition of the ectodermal (crural) glands and of the salivary 

 glands (which perhaps arise from the mesoderm), also by the double 

 rudiment of the heart and by the formation of the genital glands ; 

 but unfortunately our knowledge of the ontogenetic processes in 

 these cases is not sufficient to raise conjecture to the level of 

 certainty. The adult animal is better understood, and in it the 

 constitution of the mouth-parts, the segmentation of the nervous 

 system, the structure of the eyes, the presence of the Malpighian 

 vessels, as well as the condition of the blood-vascular system and 

 the ._ body-cavity prove without doubt the near relationship of the 

 Myriopoda to the Insecta. By far the most striking point of agree- 

 ment is afforded by the tracheae, which are constituted exactly like 

 those of the Insecta. If w T e now ascribe great importance to a 

 point which was not considered applicable in a comparison with 

 the Arachnids, it is because a derivation of the long and fairly 

 homonomously segmented Myriopoda from forms like Peripatus 

 already provided with tracheae is naturally suggested, while such 

 a^derivation of the Arachnida is met with great difficulty, as has 

 already been shown more in detail (p. 110). 



In spite of the great agreement existing between the tracheal 

 system of the Myriopoda and that of the Insecta in the adult 

 stage, one fact in connection with the former seems to suggest 



* [See footnote, p. 216.— En.] 



