290 



INSECTA. 



in the Muscidae must be regarded as secondarily acquired, when we 

 take into consideration the condition of other nearly related forms. 



We have as yet no certain data to help us in discussing the 

 question of the physiological significance of the germ -envelopes. 

 Although the increase of the yolk-absorbing surface may have been 

 of importance for the development of the invaginated germ-band, 

 this consideration does not help to explain the development of the 

 amniotic folds that have grown over the germ-band. In the latter 

 we seem to see the influence of an ontogenetic tendency which led 

 to the germ-band being separated from direct contact with the inner 

 surface of the chorion (or vitelline membrane). This may have 

 afforded greater protection against certain mechanical injuries, 

 perhaps also against the danger of desiccation or adherence. The 

 latter hypothesis seems to receive special support from the fact that 

 eggs with degenerated embryonic envelopes (Cecidomyia, Tachina, 

 Muscidae) are, in consequence of the nature of their surroundings, 

 less exposed to this danger. All these conjectures, however, afford 

 little satisfaction. 



4. Development of the external form of the Body. 

 A. Segmentation. 



The first traces of segmentation are found very early in the 

 germ-band of the Insecta, which becomes divided up by superficial 

 transverse furrows into a number of somites. This segmentation, 

 in the form of consecutive metameres, may appear as early as the 

 very beginning of gastrulation (Hydrqphilus, Kowalevsky and 

 Heider, Fig. 134 -4 and B, p. 270, and Chalicodoma muraria, 

 Carriere, No. 13, Fig. 156, p. 315). The transverse boundaries 

 of the segments then extend not only over the middle plate (p. 310), 

 from the invagination of which the lower germ-layer arises, but 

 laterally over the lateral plates (Fig. 156, s), which become the 

 ectoderm of the germ-band. These transverse furrows owe their 

 origin to the alternate thickening and thinning of the epithelium, 

 which at this stage forms the embryonic rudiment, the furrows 

 corresponding to the thin areas. It follows that, in the forms just 

 enumerated, after gastrulation has taken its course, not only the 

 ectoderm, but the lower layer also, is segmented. 



Heider (No. 38) maintained in the case of Hydropldlus that the 

 first indications of segmentation even precede gastrulation. Similar 

 transverse zones of the blastoderm have been observed by "Wheeler 

 (No. 95) in Doryphora and by Graber (No. 30) in Lina, but these 



