hall: MESONEPHKOS A>^D MULLERIAX duct in AMPHIBLi. 47 



The mesonepliric blast ulae are very conspicuous, being considerably 

 largei' than in Stage VII. They are more rounded iu cross-section, and 

 the radial arrangement of the nuclei, wliich began to be noticeable in the 

 last stage, is well established. This radial arrangement is seen not only 

 in cross-sections, but in frontal and sagittal sections as well, which proves 

 that the nuclei all point toward a common centre. In frontal sections 

 the blastulae are seen to be elongated masses, two or three times as long 

 as they are broad, each one just touching its neighbors. AVhile the 

 median wall of the blastula appears nearly straight and is parallel with 

 the long axis of the body, the lateral wall appears quite convex, project- 

 ing outward over the Wolffian duct. This icall is also thicker than the 

 median and has more elongated nuclei.^ 



It is a point of importance that in this stage the enlargements which 

 I have termed " blastulae of the second order " are no longer to be dis- 

 tinguished by their size. They have overtaken those of the first order 

 in their development, and the two sets are henceforth indistinguishable 

 except by their position, and even that criterion does not long serve to 

 distinguish them, 



Tlie Dorsal Extent of the Somatic and Splanchnic Layers of the 



Lateral Mesoderm. 



Before proceeding further with the description, I wish to make a 

 slight digression to discuss the question of the dorsal extent of the 

 somatic and splanchnic layers of the lateral mesoderm. The study of 

 such a stage as that represented by Figure 19 would lead one to think 

 that the junction of the two layers lay directly over the point marked 

 coel. It really lies a little mesad of the duct, at the point designated 

 njyh'stm. In order to justify this statement it is necessary to revert to 

 the description of Stage II. It was there shown that the germ cells 

 were differentiated from both layers of the lateral mesoderm in the 

 region of their junction with the corresponding layers of the mesomer. 

 Iu all of the later stages the mesomer (represented by the mesonephric 

 blastema or blastulae) has remained closely connected with the germ-cell 

 mass and has migrated with it toward the median plane of the body. As 

 the morphologically dorsal angle of the body cavity must be at the junc- 

 tion of mesomer with lateral mesoderm, it lies, iu such a section as that 

 represented by Figure 17, in the germ-cell mass near the point nph'stm. 

 How, then, has the apparent dorsal angle in Figure 19 {coel.) arisen? 



1 This characteristic, so noticeable in frontal sections, is shown onlj to a slight 

 extent in the particular cross-section figured (Fig. 19). 



