168 DEVELOPMENT AND REDUCTION OF THE TAIL 



SERIAL DESCRIPTION OF EMBRYOS. 



Embryo No. 836, 4 mm. Greatest Length. 



This embn^o represents the earliest stage studied in this investigation, and a dia- 

 grammatic profile reconstruction of it is shown in figure 1, plate 1. It was sectioned 

 transversely and a series of models of it were reconstructed under the direction of Professor 

 Evans. These were available for comparison and were of particular value in orienting 

 the sections in the lumbo-sacral region, where the caudal extremity curves so that it lies 

 transverse to the axis of the trunk. The embryo contains 28 somites and a long meso- 

 dermic remnant which extends about one-third of its length beyond the caudal end of the 

 spinal cord. The chorda dorsalis runs along the ventral surface of the spinal cord in close 

 apposition to it and terminates before reaching the caudal end of the central canal; whereas 

 the caudal gut extends farther down towards the tip of the tail, as indicated in figure 1. 

 The caudal extremity of the embryo consists of a mass of germinating cells into which the 

 ends of the spinal cord, the chorda dorsalis, and mesodermic remnant all merge, their out- 

 lines becoming entirely obliterated. The primordium of the caudal artery extends to the 

 tip of the tail. 



Embryo No. 786, 4 mm. Greatest Length. 



This specimen has 30 somites and a mesodermic remnant ; the latter consists of a long 

 cord joined to 4 globular masses, the last of which is slightly longer and thinner than the 

 others. The counting of the somites in small embryos is always very difficult, as pointed 

 out by Keibel, especially when the material is not well preserved. It was easy, however, 

 to make out the vesicula auditiva in the sagittal sections of this specimen, and caudo- 

 dorsal to it the glossopharyngeal nerve. The ganglionated vagus nerve in turn is situated 

 caudal to this, being surrounded by the internal jugular vein, which curves around its 

 caudal margin. The first cervical somite lies dorsal-caudal at a distance of 180 to 200 /u 

 from the vagus ganglion. The embryo has 3 occipital somites. 



Embryo No. 810, 5.5 mm. Crown-Rxjmp Length. 



A graphic reconstruction of the caudal end of embryo No. 810 is shown in figure 2, 

 plate 1, in which the structures are diagrammatically straightened out. In reahty the 

 caudal end is bent to the right and towards the front, its tip nearly reaching the right side 

 of the face. The first cer\'ical ganglion is about one-third the size of the second and lies 

 close to the Froriep ganglion, which is situated at the dorsal side of the bow-shaped trunk 

 of the accessory nerve and is particularly large on the left side. There are 32 spinal ganglia, 

 the last 2 being small. I was able to count on the right side 37 somites and a mesodermic 

 remnant ; on the left side only 36 somites and a remnant. In the cranial portion of the 

 mesodermic remnant there seem to exist potential somites, the outlines of which, however, 

 can not yet be made out. Its caudal part consists of a small, bell-shaped remnant. The 

 entire remnant is about the length and size of 4 somites. The mesoderm in the caudal 

 end of the embryo is primitive in character and can be seen dividing into its parietal and 

 visceral layers. Embedded in these can also be seen the caudal gut and a simple vascu- 

 lar plexus. The spinal cord, with its central canal, extends to the caudal end of the meso- 

 dermic renmant. The caudal gut, which extends from the cloaca to the end of the tail, 

 is beginning to disappear in this embryo at a point 200 n caudal to the cloaca and 657 m 

 from the extreme end of the tail between the thirtieth and thirty-first somites, as is indi- 

 cated in figure 2, plate 1. In figure 31, plate 2, these conditions are shown more in 

 detail. At the site of the beginning degeneration the ends are sharply pointed, but are 

 still connected by a cell-strand. The shorter, cranial portion of the caudal gut opens into 

 the cavity of the cloaca. The longer, caudal portion ends blindly at both extremities, 

 the caudal end merging into the mesodermic cell-mass and uniting with the caudal end of 



