362 EMBRYOLOGY. 



attached to the median side of it, short transverse branches, which 

 we shall designate as mesonephric tubules. 



Whereas there can no longer exist any doubt about the origin of 

 the mesonephric tubules from the middle germ-layer, the statements 

 concerning the method of their formation are still at variance with 

 one another. In accordance with the fundamental investigations 

 of Semper, it was generally believed that the mesonephric tubules 

 either were eEaguaated4n m«tameric sequence along the dorsal wall 

 of the body-cavity out of its epithelial lining, or grew forth as 

 originally solid buds, as glandular sacs do from the outer or inner 

 germ-layer. 



This view, according to the more recent investigations of Sedgwick, 

 WiJHE, and RtJcKERT for the Selachians and the three higher classes 

 of Vertebrates, is no longer adequate. In these cases the development 

 of the mesonephric tubules is intimately connected with that of the 

 primitive segments. When the latter begin to be more sharply 

 separated from the lateral plates, there arises at the place of con- 

 striction a narrow stalk, which maintains for a time a connection 

 between the two parts (fig. 204 vb). In the Selachians it possesses 

 a small cavity, which unites the ca^^ity of the primitive segment with 

 the body-cavity. In the Amniota it is solid (fig. 200). Inasmuch as 

 the successive cords (stalks) are here closely pressed together, they 

 appear like a continuous cell-mass interpolated between primitive 

 segment and lateral plate, and have been previously mentioned under 

 the name of the middle plate. On account of its relation to the meso- 

 nephric tubules, the middle plate is also designated as mesonephric 

 blastema. The mesonephric duct, split oS from the outer germ- 

 layer, is to be seen taking its way on the lateral side of and close 

 to the connecting stalks of the primitive segments. Each of the 

 connecting stalks, which Ruckert names at once nephrotome, — in 

 contradistinction to the remaining parts of the primitive segment, 

 which produce the muscle-plate (myotome) and the cell-material for 

 the skeletogenous tissue (sclerotome), — is afterwards metamorphosed 

 into a mesonephric tubule. Whereas one of its ends remains con- 

 nected with the body-cavity, the other becomes separated from the 

 primitive segment (fig. 205 uk^), then applies itself closely to the 

 mesonephric duct, fuses with the wall of the latter, and opens into it. 

 In the diagram (fig. 205) the detachment of the connecting stalk 

 from the primitive segment is shown on the right, the fusion of the 

 detached end with the mesonephric duct on the left. According to 

 this whole process of development the mesonephros is from the very 



