HABITS, ETC., OF CEREBRATUHJS LACTEUS. 175 



counterpart of the spin-threads sent out from the polar 

 bodies and from the blastomeres during segmentation. Some 

 at least of those spin-threads were contractile^ while these 

 mesenchyme spin-threads actually develop into typical 

 muscles. The same thread which at first exhibits a visible 

 streaming of its cytoplasm afterwards becomes contractile 

 and shortens just as visibly. 



2. The transition from pseudopodinm to muscle- 

 fibre consists essentially in a striation or fibrilla- 

 tion of a portion of the cytoplasm, and the 

 gradual assimilation of the non-f ibrillated re- 

 mainder. 



Such a striation of cytoplasm is nothing new. It has been 

 observed by Flemming and many others in the living cells 

 of cartilage, epithelium, connective-tissue and other animal 

 cells, in which the fibrillse form a sort of loose netwoi'k. It 

 has been seen by G. F. Andrews in living cells of many 

 Protozoa and Metazoa (4, 5), where it was plainly correlated 

 with contractile activities. In preserved material striation 

 is shown in the pancreas cells of Necturus (51) and in the 

 beautiful preparations of ciliated cells from Anodonta and 

 Cyclas (20). 



But the very best examples are those given by Heidenhain 

 (24) of the radiating system of fibres in the leucocytes or 

 wandering cells of the salamander, and by Zimmermann (52) 

 of similar radiating systems in the pigment cells from the 

 epidermis of fishes. Here the cytoplasm not only becomes 

 fibrillar, but both authors believe it to represent the con- 

 tractile elements by means of which the cells change their 

 form and creep about (61). 



In all these cases the cytoplasm becomes fibrillar inside 

 the cell, but that ought not to exclude the possibility of its 

 repeating this tendency outside the cell in the pseudopodia. 



Conn states (17) that in the mesoderm formations of 

 Thalassema the cells which are budded off from the entoderm 

 near the blastopore do not become true wandering cells, but 

 are immediately transformed into muscles. Each muscle is 



