Membranes— Segmentation 93 



as a circular thickening of the blastoderm in front of 

 the head region of the embryo, and grows over the 

 egg, covering its whole surface except the poles, and 

 then shrinks again as the embryo increases in size. 

 In the embryos of Springtails and of some flies, the 

 amnion and serosa are not formed at all, or are repre- 

 sented only by vestigial folds (53, 58 b, 64). 



Segmentation. — That the primitive streak is des- 

 tined to develop into an insect is shown at an early 

 stage by the appearance of transverse furrows which 

 divide it into segments, the segmentation beginning 

 at the front end where the primitive band spreads out 

 on either side to form the rounded head-lobes. Be- 

 tween these a central inpushing of the ectoderm 

 marks the position of the future mouth, and ultimately 

 gives rise to the fore-gut (the gullet, crop and 

 gizzard of the insect). A similar central inpushing 

 of the ectoderm at the hinder end of the primitive 

 streak forms the rudiment of the hind-gut. The 

 number of segments varies as observed in different 

 insect embryos, but the normal number may be re- 

 garded as twenty-one, of which six belong to the 

 head, three to the thorax and twelve to the abdomen. 

 On each segment, except the first and last, a pair of tiny 

 limbs or appendages bud out. The segments of the 

 head are (l) the head-lobes {protocerebral segment) 

 situated in front of the mouth ; (2) the antennal 

 {deiitocerebrar) segment whose appendages develop 

 into the feelers, appearing first behind the mouth but 

 moving in front of it as growth proceeds ; (3) the 

 intercalary (tritocerebral) segment, whose appendages 

 have only been observed in a few embryos and dis- 

 appear at an early stage ; (4, 5, 6) the mandibular 

 and two maxillary segments whose appendages form 

 the three pairs of jaws of the adult insect (fig. 60 -D, 

 E). The three thoracic segments are more or less 



