August 12, 1921] 



SCIENCE 



133 



likely to do more harm than the more or less 

 ineffective artificial protection of a few indus- 

 tries would do good. It is to be hoped, there- 

 fore, that institutions in which such scientific 

 research is carried on will be placed beyond the 

 effect of the new restrictions on import. — 

 Nature. 



SPECIAL ARTICLES 



THE PRACTICAL SIGNIFICANCE OF THE REVO- 

 LUTION OF THE EMBRYO IN APHID EGGS 



In 1916 W. F. Turner^ and the writer pub- 

 lished a paper on the green apple aphis, in 

 which certain studies on the embryology of 

 the species were reported. Studies on other 

 species have since been completed and it seems 

 now worth while to point out the important 

 bearing that certain phases of the embryonic 

 development have on the hatching of the egg 

 under varying conditions. This seems espe- 

 cially urgent from the viewpoint of control 

 in the egg stage. 



As pointed out by Baker and Turner, the 

 egg envelopes in the three common apple 

 species, pomi, malifolice and prunifolios (avenas 

 of American authors) are two in number, the 

 chorion which is thick and glossy black in 

 color and the vitelline membrane which is 

 delicate and transparent. At the time of 

 deposition the egg is embedded by the female 

 in a viscid material which serves to hold it 

 in place on the twigs. This soon hardens and 

 firmly fastens the egg in its location. This 

 material covers irregularly all eggs and serves 

 not only to cement them to the twigs but also 

 as a protection for the chorion during the 

 winter. It no doubt corresponds, in the 

 Aphiinas to the waxen coating with which 

 the females of the Eriosomatinae cover their 

 eggs. A somewhat comparable condition is 

 met with in other insects in which a glutinous 

 cap covers the micropyle-area and may extend 

 as an envelope over the greater part or even 

 the entire egg. 



The eggs of all three species when laid are 

 of a somewhat greenish color and this changes 

 ultimately to the glossy black of the winter- 



1 Journal of Agricultural EesearcJi, Vol. V., 

 No. 21. 



ing egg. This change in color coincides with 

 preliminary embryonic development. This 

 usually occupies about five day's time. Eggs 

 which are infertile or in other ways abnormal 

 do not change color in the usual way. In 

 fact most infertile eggs are not of the normal 

 green color when laid but have an orange or 

 brownish tinge which may darken with age. 



One of the most interesting phases in the 

 development of these aphids is the resting 

 stage of the embryo. All eggs, no matter 

 whether laid early or late, reach this same 

 stage for wintering. This is the normal 

 dormant condition. The embryo lies in the 

 center of the egg with its cephalic portion 

 toward the posterior pole. The caudal half 

 of the abdomen is reflexed dorsad in such a 

 manner as to include the ovarian yolk. Seg- 

 mentation is well marked and the formation 

 of the appendages has begun. The stomato- 

 deum and proctodeum are present while the 

 formation of the mesenteron has begun. The 

 genital rudiments are separated into two 

 groups but the ovarian yolk is not yet divided 

 and at the jwsterior pole lies the polar organ. 



In this condition the embryo, especially of 

 pomi and malifolice, remains until early 

 spring and it must remain in this condition 

 throughout the winter until normal growth is 

 resumed. Attempts to force the eggs to their 

 spring development are without success. 



In the early spring development is resumed. 

 This takes place in the vicinity of Wash- 

 ington, about the middle of March with pomi 

 and malifoliw. This development is accom- 

 panied by a movement of the embryo through 

 the yolk toward the posterior pole until that 

 portion of the amnion which lies above the 

 head comes in contact with the serosa at its 

 junction with the polar organ. The two en- 

 velopes then rupture here and the embryo re- 

 volves. This is a most important period in 

 the development of the species and the time 

 of this revolution is of great significance in 

 understanding certain results which have been 

 obtained by different workers. 



It has been shown by Baker and Turner 

 that an elevation of temperature before revo- 

 lution is fatal to the embryo. It is also im- 



