sec 



252 INVERTEBEATA chap. 



the protocephalio region, on the contrary, the first of the three head 

 segments, the acron, is clearly marked off. 



In the next stage the definitive segmentation is completely 

 attained. The head region is divided into three segments — the 

 acron, the antennary, and the intercalary; the jaw region into 

 mandibular, first maxillary, and second maxillary segments ; the 

 thoracic region into three segments, and the abdominal region into no 

 less than eleven segments. As soon as this definitive segmentation is 

 attained, the appendages begin to make their appearance ; first the 



antennae, later the three pairs of 

 jaws, and then the three pairs of 

 legs. All these appear as broad, 

 slightly marked elevations, fading 

 out into the general level of the 

 segment near the mid-ventral hne, 

 |, but becoming marked towards the 



9^ edges of the embryonic area. 



FIG. 20o!lLongitudmal section through ^^ front of the Stomodaeum, 

 the ahdommal appendage of the em- which has nOW appeared in the 

 bryo of Donacia crassipes to show its region of the first Segment, the lab- 

 glandular character. (After Hirsehler.) ^^^ ^^ ^^^ ^^^^^ ^j- j^^^ 

 gl, gland cells ; sec, secretum. i t j_ ■ i 1*1 



marked transverse ridges, which 

 subsequently unite with one another. Two very small elevations 

 appear near the middle line behind the stomodaeum, and these sub- 

 sequently unite to form the apical portion of the median projection 

 called the hypopharynx. The latter belong to the intercalary seg- 

 ment, but the basal part of the hypopharynx is formed from the 

 sternal regions of the jaw segments. Of the abdominal segments 

 only the first develops an appendage, which appears as a low rounded 

 elevation on each side. Sections show that it is really a shallow 

 cup hned with columnar cells, and that it secretes a chitonous plug, 

 which fills up the cavity of the cup and projects on the outside. 

 This appendage, as development proceeds, gradually disappears, the 

 elevation sinking gradually to the general level of the segment again. 

 At a later stage the appendages have grown in length and certain 

 of them undergo rotation and other changes. The antennae shift 

 forwards so as to lie at the sides of the stomodaeum, and they are 

 eventually situated in front of it. The axes of the jaw segments, 

 instead of being at right angles to the long axis of the body, are 

 inclined forwards towards the mouth, and the basal portions of the 

 second maxillae fuse to form the labium. The labrum is composed 

 of an unpaired basal piece and two distal projections, and whereas it 

 was at first directed forwards it is now reflected backwards so as to 

 cover the mouth. The rudiments of the legs have grown longer and 

 indications of their division into joints have appeared. At the same 

 time, near the mid-ventral line, at the base of each appendage, a 

 thickening of the ectoderm is seen which is the rudiment of the 

 corresponding ganglion. 



