THE FORMATION OF METAMERES. 



227 



The more posterior thoracic metameres are cut off from the anterior margin 

 of the anal plate, as rather narrow bands or ridges, s^ and s^. Later their pe- 

 ripheral ends join the germ wall and spread rapidly in a lateral direction. The 

 cheliceral metamere, s^, appears at a relatively late period between the cephalic 

 lobes and the second thoracic metamere. (Fig. 125, D^.) 



The sequence in the development of the abdominal metameres is similar to 

 that of the thoracic. First, the operculum and first gill appear, then a pause, 

 followed by the remaining gills in order. Finally the chilaria appear at a late 

 period in front of the operculum. (Figs. 141, 142.) 



126 



127 



Figs. 126 and 127. — Diagrams to illustrate the methods of cleavage, gastrulation, and the growth of the germ layers 

 and the cephalic navel in a yolkless egg, and one with yolk. The yolk is shown in black. 



The further growth of the metameres and appendages is shown by the figures 

 and need not be described in detail. We would, however, call attention to the 

 fact that there is a period when the second, third, and fourth thoracic metameres 

 are especially conspicuous, and the appendages first to appear are formed on these 

 metameres. (Fig. 140.) During this period, the fifth and sixth metameres and 

 the whole abdominal region may be deeply depressed, sometimes forming a deep 

 infolding, on the floor of which the abdominal appendages are developed. All 

 the thoracic appendages have appeared by the tenth or eleventh day. 



4. The Gastrula. 



Returning to stage A. We have seen that two distinct median germinal 

 spots appear near the summit of the primitive cumulus. 



The anterior one (Figs. 123 and 124, a.c.) comes to lie near the center of the 

 future procephalic lobes. It soon disappears from surface views, but remains 

 visible in sections as a mass of "yolk cells,"i28 D^. a.c. It augments by division, 

 and by the migration of new cells from the surface, up to stage G and H, when 

 under favorable conditions it may be seen even in surface views, as a deep lying 

 cloud of cells, now showing the peculiar histological characters of degeneration. 

 (Fig, 140, G.a.c.) 



