338 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



embryo becomes folded more and more toward the ventral side, until 

 the curvature of the embryo becomes completely reversed (Figs. 290, 293) . 



After the dorsal organ has passed into the yolk, the lips of the dorsad- 

 growing mesoderm, which on either side unite the splanchnic and somatic 

 layers, gradually envelop the yolk, and finally meet each other along the 

 middle line of the dorsum. By the fusion of these two mesodermic lips 

 a solid cord of cells is formed, occupying the median longitudinal line of 

 the back indicating the first foundation of the heart (Fig. 294). The 

 cord later acquires a lumen. This observation of Patten is at variance 

 with what usually takes place in insects. 



Strindberg (1915) in his account of Sialis, takes exception to Patten's 

 interpretation as given above of the development of the mid-gut epi- 

 thelium in Neophylax, expressing his belief that this layer of cells arises 

 from anterior and posterior mesenteron rudiments as in Sialis. The 

 subject is more fully discussed in Part I. 



LEPIDOPTERA 



The Yellow Bear (Diacrisia virginica Fabr.) 



Diacrisia virginica, which is widely distributed over the United 

 States, is quite common in its range. Males are frequently taken at 

 light; the females, more rarely. The spherical eggs which are laid in 

 clumps upon the host plant hatch in about six days at normal autumn 

 temperature. In captivity the moth lays her eggs in the evening of five 

 or six successive days until several hundred are deposited. 



About 40 minutes after deposition, the egg nucleus is to be seen in 

 the anaphase stage of the first maturation division lying a little to one 

 side of the upper pole of the egg and embedded in a mass of the periph- 

 eral cytoplasm (Fig. 2955). At this time, elimination of chromatin 

 takes place in the form of an equatorial disk, the amount eliminated 

 being variable, leaving a constant amount in the daughter cells. This 

 process agrees with Seller's account (1914) for Lymantria dispar and 

 Orgyia antiqua. 



The chromosomes are oval, more or less uniform in size, estimated 

 to average 0.5 micron in length, thus resembling those of Lymantria dispcr 

 as figured by Seller (1914). The axis of the spindle is perpendicular to 

 the surface of the egg. The sperm (Fig. 2955) at this time is located 

 apparently directly under the upper pole about 90 microns from the egg 

 surface. It has the form of a thick, straight rod, in an island of cyto- 

 plasm. In the next two hours the second maturation division takes 

 place, and the female pronucleus migrates inward and fuses with the 

 sperm. The first polar body does not divide completely, though a spindle 

 is formed and an irregular division of chromosomes occurs. The second 

 polar body lies in contact with the first. 



