164 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



genetic type, the differential features of successive offspring change at 

 different times and rates. The first to change are the color of the 

 antennae and the color and size of the tibial sensoria, then the body color 

 and reproductive system. Within the reproductive system the colleterial 

 glands and seminal receptacles change sooner than the ovarioles. How- 

 ever, when the mother is induced to revert to the production of gamic 

 instead of parthenogenetic offspring, this series of changes instead of 

 occurring in the same order occurs in the reverse order contrary to 

 expectations based upon the time of determination hypothesis. This 

 phenomenon remains unexplained. 



HALLEZ'S LAW OF ORIENTATION 



This "law" formulated by Hallez (1886) is based on the observation 

 that the mature egg within the ovary lies in such a position that all three 

 axes of the presumptive embryo are oriented coincidentally with those of 

 the mother. From this it follows that the embryonic axes are deter- 

 mined in the egg before laying. Although this is certainly a general 

 rule, it is not universal. In the radially symmetrical egg of the bug 

 Pyrrhochoris, Seidel (1924) reports that the longitudinal embryonic axis 

 may vary from being coincident with the longitudinal axis of the egg to 

 being transverse to it. Also there are certain insects (e.g., Melanoplus, 

 Slifer, 1932a) in which it is reported that the embryo begins to develop 

 in the reverse position with the head toward the micropylar opening at 

 the posterior end of the egg but that the "normal" position is attained 

 by a later reversal of the embryo during blastokinesis. Pteronarcys 

 proteus, likewise, according to Miller (1939), does not conform to the law, 

 the prospective position of the embryo being at right angles to the 

 longitudinal axis of the ovariole. Neither does blastokinesis alter this 

 relation, even though the frontal (horizontal) plane of the embryo changes 

 from a position parallel to the venter of the egg to one parallel with its 

 lateral walls. 



Seidel (1929a) reports that frontal doubhng, induced by the produc- 

 tion of splits in the egg at the beginning of cleavage and resulting in the 

 formation of mirror-image symmetrical twins within the egg, indicates 

 the determination of a dorsoventral axis (as well as other axes) at this 

 early stage in the development of Platycnemis. Mirror images result 

 from the inversion of one of the three axes in the formation of one partner; 

 this presupposes the existence of polarity at least in the inverting plane. 

 Frontal and lateral doubling have also been induced by sphts in the germ 

 band of Tachycines (Krause, 1934). 



References 



Extended bibliographies are given in two reviews on experimental embryology: 

 one by Seidell (1936), the other by Richards and Miller (1937). 



