NORTH AMERICAN ODONATA. 193 



symmetrical halves — the lateral lobes (Seitenlappen), which eventu- 

 ally take part in the formation of the head. The thick part of the 

 wall of the invaginatecl tube, together with the lateral lobes, forms 

 the germinal-band (Keimstreif ). 



While the above-described changes have been taking place in the 

 germijial band, the cells of the rest of the blastoderm, except at the 

 anterior pole, have become flattened and surrounded by an inter- 

 cellular substance. The most external layer of cells of the lateral 

 lobes undergoes the same change, unites with the similar cells around 

 the mouth of the invagination, and so closes the open end of the 

 amnion cavity. The egg is thus enveloped by a membrane — the 

 serosa — which is entirely free and unconnected with the amnion,* or 

 the germinal band. Meanwhile the yolk, which lies almost every- 

 where between the serosa and the embryo, has been divided into 

 spheres. 



The deep (hind) end of the germinal band now curves so that the 

 hind part of the abdomen becomes folded ventrally upon itself 

 Three pairs of swellings appear on the amniotic side of the germinal 

 band, the beginnings of the three pairs of legs. In front of these 

 (in this stage of development, nearer the posterior pole of the egg) 

 three similar pairs of swellings soon appear, which will be the two 

 pairs of maxillae and the mandibles (fig. 33D). The length of these 

 swellings decreases from behind forwards. As soon as these swellings 

 are clearly recognizable, two distinct layers can be seen to be form- 

 ing in each — an outer, skin layer and an inner, nerve-muscle layer ; 

 each layer of each swelling is continuous with the similar layer of 

 the swelling in front and behind. As they increase in length, the 

 swellings are directed towards the hind end of the embryo. Soon 

 after the formation of the mandibular swellings, a similar antennal 

 pair appear in front of them, and in front of the antennae a single, 

 median, unjmired swelling, the front- Jiead (Vorderkopf). Antennae 

 and front-head show the same two layers as the other extremities. 

 Behind the front head an invagination takes place — the beginning 

 of the fore-gut. The front-head eventually gives rise to the clypeus 

 and the labrum, and fuses with the lateral lobes which form the 

 remainder of the head, excepting, of course, the mouth-parts. 



The embryo now stretches itself so that the lateral lobes, instead 

 of lying near the ventral surface of the egg, approach its posterior 



* The aiunion is the viscerales Blatt of Brandt, and the serosa is his parietales 

 Blatt. 



TEANS. AM. ENT. SOC. XX. (25) AUGUST, 1893. 



