66 PATTEN. [Vou. XII. 
erating cells, to one in which it consists of a thin layer 
of cells deeply invaginated in the centre, and from the in- 
vaginated part alone arises a cloud of scattered yolk nuclei. 
BeOS hie.73. 
Whether the depressions in Figs. 72, 79, and 81 represent 
the oral and anal depressions, or the pits left after the degenera- 
tion of fused and invaginated appendages, or of one or more 
segments, as in Pl. VI, Fig. 61, cannot be determined. 
In some cases, as in Figs. 69 and 73, where the outline of 
the axis of the embryo may be faintly distinguished, there is 
no cephalic cloud of cells visible. 
It would thus appear that degeneration may carry old embryos 
back to a state resembling that seen in very young embryos, 1.e. 
one where tt consists of a cluster of proliferating cells at either 
end. 
These facts seem to indicate that the body of the embryo ts 
not a single organic unit, such as it would be if it were an 
elongated gastrula with fused lips, but rather one of a double 
origin. 
We may regard the head Azlage, from which arise the 
cephalic lobes and oesophagus, as representing the remnants 
of a trochosphere, and the posterior Anz/age as the primitive 
trunk which arises from the trochosphere as a bud-like out- 
growth. I have already shown elsewhere,! that the origin of 
the stomodaeal nerves in Limulus supports such a view. 
VI. Fission. 
All the cases of fission that I have seen in Limulus affected 
embryos in which the thoracic appendages were well developed, 
or if they were not present it was evident that their absence was 
due to degeneration. 
The multiple embryos must necessarily, from the methods of 
obtaining them, have been well advanced ; and any abnormality 
in the early stages is easily overlooked among the hundreds of 
eggs that one must examine in order to find them. Neverthe- 
less it seems probable that fission does not usually begin till a 
1 Morphology and Physiology of the Brain and Sense-Organs of Limulus. 
