The development of the urinogenital organs of the lamprey. J7 



which lies behind the section represented in Fig. 2 and therefore 

 nearer the middle of the somite, the mesoderm is distinctly constricted 

 at the point s on the ectodermal side and also at a corresponding 

 point on the entodermal side. These constrictions tend to separate 

 the myomere from the lateral mesoderm. The latter widens somewhat 

 before passing over into the flattened parietal mesoderm which runs 

 down over the sides of the yolk. Several nuclei are seen both in the 

 middle of this widening and in the middle of the myomere. Fig. 4 

 passes through the middle of the segment and shows the cœlom which 

 has been cut into two cavities by the constriction at 0, one (the 

 myocœle) lying in the myomere, the other (the nephrocœle) in the meso- 

 dermal widening lateral to the constriction. The cells, both in their 

 nuclei and in their shape, show a decided tendency to arrange them- 

 selves radially round these two cavities as centers. Fig. 5 represents 

 a section behind the middle of the somite and shows that the cavities 

 have again disappeared. In Fig. 6 which traverses the middle of 

 the next posterior segment we again have essentially the same con- 

 dition as in Fig. 4, although the mesoderm is not quite so far ad- 

 vanced. A trace of the original connection between the myocœle 

 and nephrocœle is represented by a line. Fig. 8 passes through the 

 extreme posterior edge of the second segment and resembles the 

 section (Fig. 2) through the extreme anterior edge of the first seg- 

 ment. The nuclei are disappearing in two small groups, one belonging 

 to the myomere, the other to the lateral thickening of the mesoderm. 

 The conditions described for these two segments are more vaguely 

 traceable in the more posterior segments, fading away into the un- 

 differentiated mesoderm in the hind portion of the trunk. 



A somewhat clearer and more comprehensive view of the early 

 segmentation of the mesoderm may be obtained from Fig. 25, PI. 2. 

 Only the neural tube (w) and the mesoderm in the middle of the 

 trunk are cut in this section. The latter shows both the transverse 

 divisions extending out from the mesial edge to form the successive 

 segments, and the longitudinal constrictions {£) which tend to separate 

 the myomeres from the remaining mesoderm. The transverse fissures, 

 however, extend further out than the longitudinal constrictions so that 

 there are two distinct regions in the lateral mesoderm, a mesial seg- 

 mented region in which the pronephros and its duct will appear, and 

 a more lateral unsegmented region, the parietal mesoderm. In the 

 lamprey it is difficult to locate the middle, or "intermediate" plate of 

 other Vertebrates. It appears to be represented only by the very 



Zool. Jahrb. XIU. Abth. f. Morph. 2 



