THE OVUM OR EGG. 11 



It will be at once manifest, if an egg be kept under observation 

 under the microscope, that most of the colour-changes taking place 

 within the egg are very closely connected with the developmental pro- 

 gress made by the embryo. The first change, which occurs very soon 

 after the egg is laid, probably represents the transition of the egg- 

 contents from their primal homogeneous condition, to that which is 

 reached when the blastoderm layer is developed, and which is accom- 

 panied by the separation of the contents from the egg-wall. There is, 

 at this stage, sometimes, a distinct change of tint, at others, the whole 

 surface becomes completely covered with black dots. 



The second change appears to take place with the formation of the 

 germinal band, and appears to be intensified as the growth of the 

 embryo continues. This probably accounts for the general darkness 

 of the colour assumed at this stage ; dark brown, red, purple and leaden 

 are tints frequently met with, and these sometimes last for a con- 

 siderable length of time. 



The third change of colour usually exhibits an intensified form of 

 that occurring in the previous stage, except that the apex, and fre- 

 quently the base of some eggs, becomes pale again. Sometimes, 

 at this stage, the egg-shell is quite transparent, and the movements of 

 the embryo are readily observable. This is particularly the case in 

 many eggs just previous to the escape of the larva. 



Eggs also vary in their ornamentation. We have already suggested 

 that the primitive lepidopterous egg was smooth, and, at the present 

 time, with the exception of minute pitting and faint polygonal reticu- 

 lation, the majority of flat eggs are comparatively smooth. The Geo- 

 metrid egg appears to be more generally highly ornamented than that of 

 any other flat-egged family, except, perhaps, those of the Crambids. 

 There is, however, a very considerable amount of variation in the 

 detail of the ornamentation, even in this group, e.ij., the egg of 

 Hemeropldla abruptaria is covered with a network of hexagonal cells, 

 with a glistening white knob or button at each angular point, but here 

 and there heptagonal and pentagonal cells exist side by side, where one 

 of the knobs has migrated to an adjacent cell ; this is a common form 

 of variation in this type of ornamentation. In the groups with longi- 

 tudinal ribs, the number of these often varies ; thus, in the egg of 

 Catocalafra.rini the number varies from 22-27, in Polyponia eijea there 

 may be 9 or 10, in Leucuphasia sinapis 11 or 12. Chapman says that, as 

 a rule, the egg of Evgonia. polychloros has eight ribs, but that sometimes 

 there are only seven, also that Polyyonia c-album may have ten or eleven 

 ribs, and Edwards gives the same numbers for the allied P. inter royationis. 

 Chapman finds that in Pharetra (Acronycta) auricoma the number of ribs 

 varies from 57 to 60, in P. alborenosa from 41 to 45, in Triaena tridcns the 

 average number is 38, but that sometimes there are as many as 44, 

 whilst one batch of eggs of this species had from 49 to 52 ribs. In 

 T. psi the number is rarely fewer than 45, and some specimens have 

 as many as 54 ; in Acronicta hporina the number varies from 41 to 

 68, whilst in Apatela aceris the number may extend from 50 to 75. It 

 appears to be, indeed, a very general form of variation in almost all 

 eggs with a moderate number of longitudinal ribs. 



The variability in the number of ribs is almost equalled by that of 

 their arrangement. The egg of Cirrhoedia xerauipelina has 24 or 25 

 longitudinal ribs. Normally, these ribs should be alternately long 



