THE PROCEPHALIC LOBES 



545 



lobes (a). A depression next forms in what is to be the middle of 

 the forehead. '* It indicates the position of the future labrum, and 

 forms the inner boundaries of the two cephalic ganglia, which are 

 developed on either side of this depression at a much later stage." 

 Almost simultaneously with the appearance of this depression, two 

 lateral folds are formed in the trunk portion of the band, which are 

 the first indications of the maxillary and thoracic regions, the 

 abdominal portion not yet showing traces of a division into seg- 

 ments' (Fig. 525, o). The thickened outer edges of the head- 



FIG. 525. Early stages in the embryology of 

 -iiiitlinn iitrem. Fig. 1, the youngest observed 

 primitive band, the serosa not yet formed ; 2, longi- 

 tudinal optic section (diagrammatic) of Fig. 1 ; 3, the 



primitive band after the appearance of the head-fold, which is indicated at this 

 time by the more rapid growth and consequent greater breadth of the lower 

 end of embryo, x '25; 4, a young embryo after the appearance of the primitive 

 segment-folds, x 50 ; 5, a more advanced embryo, with the antennal folds dis- 

 tinctly marked off; the free ends of the primitive folds have united across the 

 embryo posterior to the antennal folds, x 50; G, ventral view of the embryo 

 with the appendages budding out, x 25 (the embryo in this stage lies dormant 

 through the six colder months of the year): nm, amnion ; in, micropylar end ; 



chorion; (/!>. primitive band; bf, brain-fold; ///, yolk; if, caudal fold; l;f 



head-fold (pro- 



cephalic lobe) ; ji.fi/. f, primitive thoracic fold ; ji.fiJ .m, primitive maxillary fold p.fij.n, primi- 

 tive abdominal fold ; al>.p. abdominal constriction ; t.c, thoracic constriction at. I, antennal 

 lobe; J/, mesodcrm ; fi.f/, head groove; >n<>. mouth: *, invagination of ectoderm to form head- 

 apodeme ; md, rudiment of mandible; m l , 1st, w 2 , 2d maxilla; T 1 -T 3 , legs: <if>./>, 1st abdominal 

 appendage; np, other appendages; tb, caudal expansion ; mj\ median furrow ; J}, primitive un- 

 paired organ (metastoimim). After Ayers. 



fold next gradually groAv in towards the median line (Fig. 525, 5), 

 and bend forward towards the region of the future mouth. 

 The rounded angle made by the posterior end of the head-fold is 

 the first indication of the antennae. The embryo is now composed 

 of four well-marked regions : cephalic, maxillary, thoracic, and 

 abdominal. The primitive band then grows much longer, the 

 primitive mouth and anus appear, and the appendages bud out, 

 and eventually the embryo revolves and appears on the ventral side 

 of the egg (Fig. 525, c). 

 2 N 



