16 WILLIAM MORTON WHEELER, 



it is constricted off the abdominal pores are formed at the posterior 

 end of the body-cavity and lead into the sinus. The urinogenital 

 papilla arises as a ventral projection of the sinus and finally becomes 

 hollow. The musculature surrounding' the sinus and rectum develops 

 from the circumjacent mesenchyma of the larva. 



Part II. Descriptive. 



1. The DeA elopmeiit of the Proiiei)hros. 



a) The Segmentation of the Mesoderm. 



The embryo of Petromyzon planeri reaches what I shall call 

 stage 1 by the fifth or sixth day of its development (at Naples). The 

 head protrudes distinctly from the surface of the yolk, the neural 

 canal is visible anteriorly and the somites are appearing in the head 

 and anterior portion of the trunk. This stage is represented in 

 sagittal section by v. Kupffer (1890, tab. 27, fig. 16) and by Goette 

 in a similar section of P. fluviatilis (1891, tab. 1, fig. 8). A trans- 

 verse section just behind the segmented portion of the embryo has the 

 appearance of my Fig. 1, PI. 1. The splanchnic and somatic layers 

 of the mesoderm are very distinct mesially but fused with each other 

 and with the entoderm laterally. Karyokiuetic figures, hke the one 

 represented in the splanchnic layer, are common, and bear witness to 

 active growth on the part of the mesoderm. The cœlome is distinct 

 next to the median axis of the embryo, but elsewhere the splanchnic 

 and somatic layers are closely applied to each other. In this stage 

 no traces of the pronephros or of its duct are to be seen. 



A little later (stages 2 and 3) the head of the embryo protrudes 

 still further from the yolk. The optic lobes are distinct and the seg- 

 mentation extends further back through the trunk. This stage is 

 reached towards the end of the sixth day in P. planeri (at Naples). 

 It is represented by v. Kupffer in his fig. 17, tab. 27, and by 

 Goette in a sagittal section of P. fluviatilis (fig. 9, tab. 1). Most 

 of my embryos of this stage are in a slightly younger stage. The 

 pronephros begins to appear where the neck of the embryo leaves 

 the rounded surface of the egg. Seven sections through two suc- 

 cessive segments in this region are shown in Figs. 2 to 8. Fig. 2 

 represents a section passing through the anterior edge of a somite 

 and taking off' only the surfaces of the cells, so that neither the 

 nuclei nor the body-cavity separating the somatic and splanchnic 

 layers of the mesoderm are to be seen. In the next section (Fig. 3), 



