ZOOLOGY AND BOTANY, MIOROSCOPY, ETC. 361 



which the nuclei derived from its fellow migrate to the circumference • 

 it then divides in the same manner as the posterior nucleus, pro- 

 ducing a long line of nuclei, which are directly invested with proto- 

 plasm, and constitute the vitelline cells. The blastoderm of the 

 ventral aspect thickens, forming the ventral plate, while it becomes 

 attenuated dorsally ; the amnion makes its appearance as a single fold 

 or prolongation of the dorsal part of the head end, consisting of a single 

 layer of cells extending round to the ventral side. Shortly after the 

 amnion is formed, the germinal plate becomes grooved by a transverse 

 linear furrow, which, after deepening, remains stationary for a time, 

 and then disappears ; it is considered by Weissmann as a gastric in- 

 vagination homologous with the longitudinal groove found in other 

 insects. The mouth and cephalic constriction then appear, and a little 

 later the proctodeum. At first sight the mesoderm seems derived 

 wholly from the so-called vitelline cells ; but in reality it is derived 

 from certain globose cells from the middle of these, behind the 

 transverse groove and near the anterodorsal aspect, which become 

 detached, and range themselves below the blastoderm as a definite 

 layer. 



The mesenteron is bounded by large granular endodermic cells 

 which inclose the remains of the yolk. 



At the moment of hatching the larva has thirteen segments ; it 

 cannot be determined whether the anterior division corresponds to a 

 single segment, or to the whole head, as in all insects but the Musciclce. 

 Three pairs of tubercles, placed behind the mouth, are the rudiments 

 of the mouth-organs : the first pair form the mandibles ; the second, 

 which become rudimentary, represent the maxillary palps ; the third 

 pair fuse into a median plate, which protects the mandibles. The 

 antenna appear as tubercles on the procephalic lobes, and remain in 

 this condition during larval life. No trace of limbs occurs on the 

 segments of the body at any stage. 



BiorMza aptera differs from Hhodites in developing a vitelline 

 membrane ; as in Bhodites, the yolk penetrates into the peduncle of 

 the egg, but it is retracted again. The first segmentation-nucleus 

 lies in a transparent mass of protoplasm situated at the anterior pole ; 

 after performing amoeboid movements, it elongates in a transverse or 

 oblique direction, and divides into the two polar nuclei, and almost 

 immediately after this the posterior of the two itself begins to divide, 

 while the anterior remains for a time inactive, and then divides ; the 

 two sets of nuclei derived thus are indistingu:shable. It appears that 

 when about 100 nuclei have been thus formed they become the centres 

 of cells, part of which move to the surface and form the blastoderm. 



The anterior cells of this membrane emit slender anastomosing 

 pseudopodia like those of Eadiolaria, which project into the space 

 beneiith the vitelline membrane, but disappear after a short time. 

 The amnion is formed as in Bhodites ; no gastric invagination has 

 been observed; the development of the segmental appendages cor- 

 responds with that of Bkodites, and here also there are no thoracic 

 appendages ; the small anterior segment of the larva represents the 

 entire head. 



