MUSEUM OF COMPARATIVE ZOOLOGY. 73 



cephalic plate. The addition of new zonites to those already existing 

 goes on raj)idly ; the two anterior ones (those of the chelicerae and the 

 pedipalpi) are cnt off from the posterior end of the cephalic plate. They 

 are late in making their appearance, and, as Balfour puts it, "lag behind" 

 the others in their development. The other zonites are developed from 

 the caudal plate. 



Soon after the protozonites are first established they form ridges which 

 reach nearly around the egg, and thus appear to radiate from the dorsal 

 region. (Compare Emerton, '72, Figs. 8, 9.) They soon undergo con- 

 centration which so shortens the thickened ridges, that together they 

 form a band about 45° wide on the ventral surface of the egg — the 

 embryonic band. Fig. 6, PI. II., gives a side view of an egg in which 

 this concentration is well advanced but not yet completed. At the same 

 time the embryonic band increases in length, thus extending in an antero- 

 posterior direction further and further around the egg. When at length 

 seven or eight protozonites are fully established, the band embraces 

 approximately two-thirds of its circumference. At about this time also 

 the rudimentary appendages begin to appear ; these mark the commence- 

 ment of the third period of growth. 



The internal condition of the egg during the second period can be 

 satisfactorily studied only by means of sections. I have made sections 

 passing through the primitive cumulus in two directions, sagittal and 

 transverse. In sagittal sections two features are conspicuous : (1) The 

 ventral surface of the egg is clearly differentiated from the dorsal surface 

 by the condition of the cells along its entire length (PI. VII. fig. 41). 

 (2) The cells in the region of the cumulus are arranged in several irregu- 

 lar layers. A thickening of the blastoderm has also arisen at the caudal 

 eminence, and there is a tendency to thicken along the ventral region 

 embraced between these two structures. 



Figure 41 is from a sagittal section of the egg represented in PI. I, 

 Fig. 3 ; the cells of the ventral side are large and rounded or oval, 

 while those of the dorsal side are much flattened. The cells of the 

 primitive cumulus (cum. pr.') are conspicuous for their size ; they are 

 loosely arranged in layers. In some cases (PI. VI. fig. 39) they are four 

 layers deep. 



Sections of eggs a little more advanced show a large number of cells 

 along the ventral-plate region, and also at the caudal thickening. 



Balfour's figure (1. c, Fig. 11) of this stage cannot be compared criti- 

 cally with my own, as he was uncertain about the direction in which the 

 egg was cut ; but from its close resemblance to my sections, I think it 



