176 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
2. Free Migration of Amaboid Bodies. This process differs from 
the preceding in that the migrating bodies are not connected together. 
It is characteristic of mesenchyme. I know that Dreyer (92, Jena. 
Zeitschr, XXVI. 359) and Sedgwick (94, Quart. Jour. Micr: Sci, 
XXXVII.) insist that the cells of mesenchyme, which are usually con- 
sidered unconnected like so many amæœbæ, are really nodal thickenings 
in an extensive mesh-work or foam-work, the intervening fluids being the, 
in some places confluent, vacuoles. Wherever mesenchyme has this 
structure, its migrations belong to the preceding class. But I believe 
there still remains a considerable residuum of cases falling under this 
head. 
This process is capable of division into two subprocesses ; viz. (a) mi- 
grating of mesenchyme out of a protoplasmio layer in order to become 
free, and (b) migrating through fluid-filled spaces. Both these processes 
are illustrated in Figure 2. Further illustrations will be found in K. & 
H., Figs. 102, 103, 170, 175-180, 
182, 186, 188-190, 207, 285, 559, 
596-598, 628, 698, 733, 809, and 
M., Figs. 121, 234, 239. 
The migratory processes named 
below are of subordinate rank to 
Nos. land 2. But, being fairly well 
marked and of considerable impor- 
tance, it is convenient to treat them 
as co-ordinate. 
Fic. 2. We may distinguish, first, move- 
ments of mesenchymatous elements 
towards and from each other, and, secondly, movements with reference to 
other protoplasmic masses, 
3. First among.the former we recognize the aggregation of migratory 
protoplasmic bodies, and here we may distinguish three sub-classes accord: 
ing to the form of the resulting body. 
a. First we have the aggregation of mesenchyme into a body with a 
chiefly linear dimension, — the formation of a thread, cord, or tubule. 
Examples of this process.in Invertebrates are seen in the formation of 
the kidney of Lamellibranchs, which seems to be laid down as a cord-like 
aggregation of mesenchyme, and in that of the thread of the yolk glands 
Fig. 2. Section of Holothurian larva showing mesenchyme migrating out of a 
layer at a, and through a fluid-filled space at 6, From H.-M., Fig. 109. 
