DESCRIPTION OF TLATES. 



Plate I. 



Fig. 1. Early stage in the formation of the blastoderiu. The nuclei, nu., 



formed from the division of the segmentation nucleus, having migrated to 



the surface of the egg. lie in the perii)lasm just outside the yolk columns, 



ylk. 



Fig. 2. Blastodisc. The division of the egg into two poles is indicated 

 by the aggregation of cells, as indicated by their nuclei, on one end of the 

 egg. Irregularly iirojecting yolk masses and fewer nuclei, mark the other 

 end. A slight, blastopore-like depression, hip., appears in the center of the 

 blastodisc. 



Fig. 3. Blastodisc, viewed from above. 



Fig. 4. Primary thickening, viewed from above. The increased thick- 

 ness of the germ layer is indicated by the closely aggregated character of 

 the nuclei. 



Fig. 5. Primary thickening, viewed from the side. The slight elevation 

 to one side of the blastopore is the caudal thickening, c. th. 



Fig. G. Germ band. The germ band, g. hd., lies in the form of a trian- 

 gular plate on the surface of the egg. It bears, at intervals, thickened, 

 transverse ridges, or segments, seg. The most posterior portion of the 

 plate is occupied by the caudal thickening. The anterior portion consists 

 of the flat cephalic plate, c. pi. 



Fig. 7. A more posterior view of the same. 



Fig. 8. The germ l)and. A somewhat earlier stage, presenting fewer 

 segments. 



Fig. 9. The ventral plate. A much later period, presenting a much 

 greater number of segments, the thoracic segments bearing rudimentary 

 appendages, ap. 



