228 s. HATTA; FORMATION OF THE 



place, a number of cells quite like the mesoblastic elements, is found 

 scattered between the epiblast and the hypoblast, being especially 

 numerous on the median ventral line of that space ; these are 

 doubtless mesenchymatons cells. 



Fig. 2 represents the section through the plane indicated by the 

 line a-a in Fig. IC ; the cell-mass on the upper corner of the lateral 

 plates (jn either side is the rudiment of the pronephric ducts and 

 the second pair of the tubules (Fig. 2 u-d.) ; the ventral free edges of 

 the niesolilast do not yet meet and there is found no cellular elements in 

 the median ventral line. The splanchnic plates consist of several layers 

 of cells, while the somatic plate is only one cell thick. Fig. 3 shows 

 a transverse section through the corresponding portion of an embryo a 

 little older. The pronephros shows considerable progress ; both the 

 somatic and the splancimic plates consist of a single layer of cells ; 

 the ventral free edges of the mesoblast do not yet meet. On the 

 median ventral line a gi-oup of cells (x) not observed in Fi«". 2 is visible. 

 This is no other than the mesench\'matous elements referred to above. 

 At the present stage, this mass of cells does not yet occupv a large 

 extent, being traceable in only two or three sections. Figs. 4 and 5 

 are selected from a series of sections through an embryo a little older. 

 Fig. 4 represents a transverse section through the slender neck portion 

 just behind the second pair (tlie hindmost at this stage) of the 

 branclunl invaginations. In this, there is no trace of mesenchymatons 

 cells in the ventral median Hne, but there are some disconnected cells 

 on the ventral tip of the mesoblastic plates, falling in between the 

 epiblast and the wall of the fore-o-nt. Fio-. 5 is the fourth section 

 behind Fig. 4, and, on account of tlie peculiar bend of the neck before 

 referred to, is cut somewhat frontally (the plane of the section is 

 shown by the line «-[mu Fig. IC). Moreover, as this is just at the 

 angle winch the slender neck portion makes with the larger posterior 



