The development of the urinogenital orgaus of the lamprey. 49 



assigned to the mesoderm because their appearance and position are 

 those of the entoderm cells in this stage. Still they lie in a portion 

 of the entoderm which becomes mesoderm with the more lateral ex- 

 tension of the latter layer. This is seen in Fig. 33 from a cross- 

 section passing through the corresponding part of an embryo of the 

 next stage (stage 3). Here the segmented portion of the mesoderm 

 {mx/l and cpl) is more definitely established and the pronephric duct 

 (d) has been cut otf from its lateral portion. The huge sex-cells (gou), 

 still made up of large, rounded, yolk-laden masses with their nuclei 

 near their centers, are now embedded in cells which must be regarded 

 as mesodermal, though they may have originated from the underlying 

 entoderm. In stage 4 essentially the same condition is found, as 

 shown in Figs. 42 and 43, taken from a section passing through the 

 point at which the pronephric duct, now provided with a lumen, opens 

 into the posterior portion of the intestine. In sagittal sections of this 

 and the next stage, the huge sex-cells form a cord extending through 

 the posterior third of the embryo, even behind the termination of the 

 pronephric duct. I have reproduced this condition in the diagram- 

 matic Fig. 62 at gon. 



Later (stage 6, Fig. 45) the reproductive cells (gon) have changed 

 their position. On either side they have moved in under the pro- 

 nephric ducts towards the median line so that they are not far from 

 meeting beneath the aorta (ao). The actual fusion of the two sex- 

 cell masses to form the single median reproductive organ of the larva 

 and adult is clearly seen in a later stage (stage 11). The cells, still 

 containing yolk granules, although these have disappeared from all 

 the surrounding tissues except the mesenteron (ent), are now flattened 

 out between the nephric lobes and the intestine. With the absorption 

 of the yolk in the entoderm cells the intestine grows smaller and the 

 sex-cells regain (Fig. 46, stage 12, gon) their spherical form. This 

 change may also be due to the pressure from either side exerted by 

 the rapidly enlarging cardinal veins. The sex-cells are still conspicuous 

 on account of their larger size and because they contain a few yolk- 

 bodies even after these have completely disappeared from the ento- 

 derm cells. The sex-cells are, therefore, of all the cells in the embryo, 

 the last to consume the yolk granules which fell to their lot as blasto- 

 meres of the segmenting ovum. Up to this stage and even sometime 

 after the yolk has disappeared from their cytoplasm, they do not 

 divide. 



Zool. Jahrb. XIII. Abth. f. Morph. ^ 



