64 PATTEN. [Vou. XII. 
clearly the stage in which the whole embryo would have been, 
had no degeneration taken place. 
Perhaps the best illustration of this is shown in Pl. VII, 
Fig. 82. The mesodermic area is relatively large, and its 
posterior margins are thickened and well advanced toward con- 
crescence. There is a very large, projecting tail lobe, like that 
in Figs. 48, 49, and 94, and yet of the body proper (which should 
have all the organs seen in Figs. 5 and 6) there ts nothing left 
but a deep, thick-walled pit, with a triangular opening to the 
exterior. 
A similar condition is shown in Fig. 83, where the undif- 
ferentiated remnant of the body forms a Y-shaped sac with an 
oval opening at its posterior end. 
Figs. 84, 85, 86, 87, and 88 are various modifications of the 
same condition. 
In Fig. 88 the thickened margin of the mesodermic area 
has broken up into the star-shaped masses of degenerating 
cells, so frequently seen in the later stages of degeneration. 
These embryos represent what is left after invagination, 
median fusion, and progressive degeneration have attacked with 
varying success every part of the body. 
Most of the embryos shown on Pls. V and VI would probably 
have reached this condition ultimately. In some of these cases 
one can distinguish here and there an appendage, or some 
other organ; but the remaining parts may be so distorted or 
misplaced that it is impossible to distinguish literally as well 
as figuratively any head or tail to them, as in Figs. 71 and 56, 
and others which we have not space to figure. 
In Fig. 81 the large posterior depression appears to be the 
remnant of the anal plate, and the three obscure pits in front 
of it, the last of three fused pairs of invaginated appendages. 
In Fig. 80 everything has disappeared except the large pit, 
situated at what appears to have been the anterior end of the 
embryo. 
Whether the huge projection in Fig. 69, Pl. VI, and Fig. 
106, Pl. IX, represents a fused and partly invaginated pair of 
thoracic appendages or a tail-like projection of the anal plate 
is hard to determine. 
