32 Hjalmar Théel, 



form and in possessing smaller pseudopodia, which are sometimes totally 

 withdrawn. These cells are true calciferous cells, and constitute the 

 matrix of the future spicules, thus corresponding to the osteoblasts in 

 the vertebrated animals. 



However, it ought to be kept in mind that not all cells which 

 first become free, are calciferous cells, because I have often, though not 

 always, seen cells detached, which were provided with large and long 

 pseudopodia and were evidently destined to perform different functions. 

 In the blastula stage these cells are by no means so prevalent, but in 

 a subsequent developmental stage they increase considerably in number. 

 Thus a very early differentiation appears to take place in the mesen- 

 chyme cells. 



The two heaps of calciferous cells soon become combined by a 

 tranverse bridge of cells crossing the ventral surface of the Blastula, 

 PL II, fig. 34—35 and PL III, fig. 37, and at about 20 or 25 hours 

 after the fecundation a very minute calcareous deposit is to be found 

 at the middle of each heap of cells. When the cells are ready to de- 

 posit calcareous matters, they contain a multitude of granules of varying 

 size and brightness, among which one or more calcareous crystals are 

 present, thus rendering the nucleus visible only with difficulty. During 

 the formation of calcareous substance, PL 1 V, fig. 76, the cells have chan- 

 ged considerably and vary in size, some being highly elongate, measuring 

 up to 0,036 mm., others remaining smaller, having a diameter of only 

 0,012 to 0,016 mm. 



The other cells of the mesenchyme, which have to perform dif- 

 ferent functions, have an irregular shape, fine granules and a distinct 

 nucleus. By degrees during the immigration they protrude finer and 

 coarser pseudopodia, which sometimes anastomose with the pseudopodia 

 of other cells, thus giving rise to a kind of network with wide meshes, 

 PL 1 V, fig. 66 — 68. I have often seen these amœboid cells arrange them- 

 selves in such a manner as to constitute an intercommunication between 

 the ectoderm and the invaginating entoderm. It is common to find in 

 young Gastrulge such cells attached by one pseudopodium to the ectoderm 

 and by another to the archenteron, which is just in a state of growth, thus 

 giving one the impression that they facilitate the process of invagination. 



As I have mentioned above, the cells in question begin the im- 

 migration during the blastula stage though sparingly, and it is of interest 

 to note that they are continuously separating and entering the blastocœl 

 during the whole gastrulation. When examining sections of Gastrulas in 



