No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 43 
owing to the fusion or reduction of organs at the extremities 
of the body, and the fullness and completeness of structure 
shown in the intermediate regions, testify to the universality 
of this double law of growth and degeneration. 
What is the cause of this mode of degeneration? It seems 
to be an exaggeration of the forces which have determined the 
form and manner of growth in normal embryos. 
Assume, for example, that the embryo of a segmented animal 
consists of a double row of independent half-metameres, placed 
with their oldest ends or #eads toward each other, and grow- 
ing laterally like an acrogenous plant. The series of organs 
thus formed, extending from the oldest or median end to 
the youngest, or lateral, extremity, such as the endoderm,! 
mesoderm, neuromere, nephridium, appendage sense-organs, 
and various somatic structures, will then appear on the half- 
somite, roughly speaking, in the order of the appearance of such 
organs throughout the animal series. And they are analo- 
gous to the metameres of a whole animal, or to the succession 
of morphological units produced in an acrogenous plant. There 
is, however, a sharp distinction to be drawn between the 
arrangement of organs on a half-metamere and the succession 
of metameres in a segmented animal. 
In the latter case we have a succession of homologous parts 
more or less modified by their position; in the former, a suc- 
cession of fundamentally different parts—a logical sequence 
of morphological structures in accordance with the genesis of 
physiological specialization. The dorsal and ventral surfaces 
are thus forever fixed as parts fundamentally different, and 
less likely to be confounded through secondary changes than 
the anterior and posterior extremities of the body. Broadly 
speaking, therefore, the ventral surface of a segmented animal 
is the oldest and most specialized, —the dorsal the youngest 
and least specialized. 
If we had to deal only with lateral growth, —z.e. growth 
at the apex of each half-metamere, and with the formation of new 
half-metameres at the posterior end, and all this taking place on 
1 The endodermic and mesodermic portions of a half-metamere are enfolded 
at an early period, and consequently do not appear to form a part of the series. 
