MUSEUM OF COMPARATIVE ZOOLOGY. 75 



alike in constitution ; some of them are stained deeply and appear homo- 

 geneous, while others are stained lighter and appear granular. During 

 the whole of this period there continue to remain in the yolk mass a 

 large number of cells, which are distributed through its substance at 

 tolerably regular intervals. There is often a comparatively small amount 

 of protoplasm enveloping the large angular nuclei of these yolk-cells, and 

 about them the yolk corpuscles are more or less definitely grouped. 



3. At the beginning of the third period the embryo still has a trans- 

 versely banded appearance as in the protozonite stage ; the concentration 

 from the sides is completed, and about six zonites are distinguishable 

 between the head- and tail-lobes. The zonites now begin to grow thin- 

 ner in the ventral median line, and at the same time their ends become 

 gradually more prominent and rounded. The small knob-like promi- 

 nences at the ends of the zonites are the rudiments of the appendages, and 

 in about two days after their first appearance (at the temperature stated) 

 the six cephalo-thoracic appendages are fully established as represented 

 in PI. II. fig. 7. The two anterior pairs of appendages are much 

 smaller than the four succeeding pairs, the latter being about equal in 

 size. The appendages thus established correspond to the chelicerse, the 

 pedipalpi, and the four pairs of ambulatory appendages of the adult. 



Simultaneous with the growth of the appendages new zonites, derived 

 from the tail-lobe, make their first appearance ; the four anterior of 

 these are very prominent, and a little later they bear four pairs of pro- 

 visional appendages (PL IV. fig. 20, ^r. app.). In this first part of the 

 third period the head plate is faintly bilobed; the tail-lobe is broad and 

 rounded. 



A ventral view (PI. IV. fig. 19) of the same egg (PI. TI. fig. 7) 

 shows a faint median furrow, which marks the thinning out of the ecto- 

 derm in the median plane after the separation of the lateral halves of the 

 underlying mesoderm. There are slight elevations just inside the bases 

 of the limbs, best seeu in optical section along the upper margin of the 

 figure ; they are the beginnings of the nervous ganglia. 



At first the appendages grow out perpendicular to the axis of the 

 body (PL II. fig. 7), but as they increase in length they curve towards 

 the median line, as shown in Fig. 8. They are now indistinctly four- 

 jointed. The central lumen, which can be observed readily in optical 

 sections of the leg, is shown by actual sections to be a prolongation of 

 the cavity of the corresponding mesodermic somite. 



At the present stage — the last part of the third period — the head 

 plate has become distinctly bilobed, a prominent upper lip composed of 



