364 EMBRYOLOGY. 



one another and are marked off from the surrounding tissue by 

 sharper contours. 



Although it is often stated that in the Amniota the mesonephric tubules 

 " are differentiated, out of" the middle plate or the mesonephric blastema, it is 

 nevertheless to be observed that this is not a case of new formation out of 

 undifferentiated cell-material. The so-called middle plate at the time of its 

 origin, in the manner previously described, is at once separated into segmentally 

 arranged cords, which are afterwards metamorphosed into the mesonephric 

 tubules. The differentiation out of a blastema is therefore here, as in most 

 cases, to be conceived of as an increase in the distinctness of already esta- 

 blished structures, which constitute a cell-mass that appears undifferentiated, 

 but only on account of our limited means of discrimination. 



In the Amphibia, Teleosts, and Ganoids the origin of the mesonephros 

 deserves to be subjected to renewed investigation from the recently acquired 

 points of view. 



Soon after their union with the mesonephric duct the individual 

 mesonephric tubules begin to grow somewhat in length, to take on 

 S-shaped curves, and to be differentiated into three regions. The 

 middle region undergoes a vesicular enlargement and is converted 

 into a BOWMAN'S capsule. Individual transverse branches from the 

 primitive aortfe, which pass along close to the mesonephros, make 

 their way to the capsules, and are there resolved into a tuft of 

 capillaries. The knot of blood-vessels, or glomerulus, now grows 

 into the epithelial vesicle, the median wall of which is pushed before 

 it and invaginated into the interior. During this process the 

 epithelial cells of the invaginated part of the wall become greatly 

 flattened, whereas upon the opposite uninvaginated side they re- 

 main tall and cuboidal. Such a structure, consisting of a vascular 

 glomerulus and the enveloping BOWMAN'S capsule, is called a Mal- 

 pighian corpuscle, an organ that is exceedingly characteristic of the 

 primitive kidney (mesonephros) and the permanent kidney (meta- 

 nephros) of Vertebrates. 



In addition to the enlarged middle part, there is to be distinguished 

 on each mesonephric tubule a narrow connecting portion, which 

 continues to increase in length, running to the mesonephric duct, and, 

 secondly, a short portion connecting with the body-cavity. The latter 

 is metamorphosed in different ways in the separate classes of Verte- 

 brates. In some, as in many of the Selachians, it retains its original 

 connection with the body-cavity even in the adult animals ; it begins 

 at the peritoneum with an opening, surrounded with ciliate cells, 

 which was discovered by SEMPER and has been designated nephridial 

 funnel or nephrostome, and which in many respects recalls the 



