176 



BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



2. Free Migration of Amoeboid 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 amoebae, 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 suhprocesses ; viz. (a) mi- 

 grating of mesenchyme out of a protoplasmic layer in order to become 

 free, and (b) migrating tlirough 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. 1 and 2. But, being fairly well 

 marked and of considerable impor- 

 tance, it is convenient to treat them 

 as co-ordinate. 



We may distinguish, first, move- 

 ments of mesenchyniatous 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. 



Exam])les 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 meseuchvme, and in that of the thread of the yolk glands 



J>- '-V~l 



Fig. 2. 



Fig. 2. Section of Holotliurian larva showing mesencliyme migrating out of a 

 layer at a, and through a fluid-filled space at 6 From II.-M., Fig. 109. 



