SEGMENTATION. 



291 



authors interpret them in another way not connected with the later 

 segmentation. 



Such an early appearance of segmentation as that found in 

 Hydrophilus and Chalicodoma must he regarded as a modification 

 of the ontogenetic processes founded on heterochrony. We shall 

 have to regard as primitive the condition found in other forms 

 (e.g., Lina and Stenobothrus, Graber, No. 30), in which the gastru- 

 lation and the separation of the lower layer take place in the 

 unsegmented germ-hand, and the division into segments only occurs 



Fig. 145. — Three stages in the development of the germ-band of Lina (after Graber). a, 

 unsegmented germ-band ; in B and C, the segmentation is recognisable in the lower layer. 

 ]:, with the rudiments of the mandibular and two maxillary segments, to which, in C, 

 the three thoracic segments and the two anterior abdominal segments are added, a', a", 

 lirst and second abdominal segments ; of, amniotic fold ; U, blastopore ; k', mandibular 

 segment ; k", k'", the segments of the two maxillae ; Id, cephalic lobes ; in, mouth ; t', t", t'", 

 first, second, and third thoracic segments ; th, extension of the germ-band in the thoracic 

 region ; u, lower layer. 



at a later stage (Fig. 145). In these forms the segmentation is princi- 

 pally noticeable in the invaginated lower layer, although probably, in 

 all cases, the ectoderm also is affected by it at an early stage. 



In the completely segmented germ-band of the Insect (Fig. 134 E, 

 p. 270, and Fig. 146 A, p. 295) we distinguish two peculiarly-shaped 

 regions, one corresponding to the anterior end and another corre- 

 sponding to the posterior end. The anterior or primary cephalic 



