MOUTH PARTS OF THE EMBRYO. 



IV— MOUTH PARTS OF THE EMBRYO 



The eggs of Bactis sp. are laid on stones in swift water in single-layered 

 masses of eighty-five to three hundred. They are covered with a thready, viscid 

 substance which causes them to adhere to each other and to the stones. An 

 individual egg (PI. I, fig. 1) is ovoid in shape and measures .152 mm. x .081 mm. 

 The chorion is yellowish and slightly pitted. 



Eggs brought into the laboratory hatched eleven days* after oviposition. 

 The ventral plate (PI. I, fig. 2, Vp) appears the second day, and differentiates 

 within twenty-four hours into primitive streak and amnion. One day later the 

 head, thoracic and abdominal regions are distinct (PI. T, fig. 3). Then the mouth- 

 part rudiments appear almost simultaneously (PI. I, fig. 5). Thoracic legs are 

 differentiated and abdominal segmentation is complete on the sixth day. Em- 

 bryonic development is finished five days later. Straightening the legs and flexing 

 the abdomen, the nymph emerges through a middorsal slit in the chorion. It 

 begins to eat immediately. 



DEVELOPMENT OF THE MOUTH-PARTS. 



Clypeo-labrum. The clypeo-labrum arises as an evagination between the 

 procephalic lobes. On the fifth day, at the time of origin of the paired mouth- 

 parts, it is a median hemispherical papilla (PI. I, fig. 5, Clp. Lm). The clypeo- 

 labral suture (PI. I, fig. 6) is evident on the eighth day. The anterior border 

 of the labrum becomes rounded and emarginate. The sensory hairs appear as 

 tiny papillae (PI. I, fig. 7) on the tenth day. They are elaborated just at hatching 

 (PI. I, fig. 8). 



Mandibles. The mandibles arise as simple cone-shaped papillae during the 

 fifth day of embryonic development (PI. I, fig. 5, md). A protuberance soon 

 appears at the inner apical region (PI. I. fig. 9, ia). A little later the mid-apical 

 region is indistinctly divided into two parts (PI. I, fig. 10, c). As the mandible 

 elongates, the outer lateral region becomes less rounded, and the inner lateral 

 portion slightly bulged. The eighth day the apical portion begins to swing toward 

 the mid-ventral line. At that time the two mid-apical parts, destined to become 

 the nymphal canines, are more distinctly defined (PI. I, fig. 11, c). Serrations 

 appear at the tips of the canines, and the molar surface is indicated on the inner- 

 lateral bulge by tiny papillae. 'Die ginglymous articulations on the dorsal and the 

 ventral surface are evident, and the inner-apical lobe is movable. The molar 

 papillae increase in size and become confluent to form transverse ridges. These 

 ridges are wedge-shaped, with the apex pointing upward. At hatching the man- 

 dible (PI. I, fig. 12) is dorso-ventrally flattened, and the molar surface (ins.) 

 inclined obliquely ventrad. The two canines (oc, ic.) each possess four teeth 

 at the tip. Molar surface, canines, ginglymous articulation (ga.), and movable 

 inner lobe (il.) are heavily chitinized. 



*Twenty-eight days in tlie cooler water of the creek. 



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