DESCRIPTION OF PLATES. 



(The description of Plate 1 will be found on the plate itself.) 



The figures on Plates 2 to 4 represent profile reconstructions of the caudal region in a series of human embryos 

 selected from the Carnegie Collection. The different structures are indicated by the following abbrevia- 

 tions: 



A. s. m. Arteria sacralis media. 



Append. Caudal extension of the terminal ventricle. 



Caud. non-v. Part of tail containing no vertebrae. 



Ch. Chorda dorsalis. 



CI. Cloaca. 



Constrict. Constriction marking off remnant of spinal cord 



which is to form the coccygeal medullary vestige. 

 Con. med. Conus medullaris. 

 Dura. Dura mater. 

 Fil. t. Filum terminale. 



Gang. symp. Sympathetic ganglionated cord. 

 Int. cau. Intestinum caudale. 

 Lig. cau. Ligamentum caudale. 

 Med. sp. Medulla spinalis. 



Med. sp. atr. Atrophic portion of the spinal cord. 

 Mem. cl. Membrana cloacalis. 



Pia. Pia mater. 



PI. vase. Plexus vasculosus. 



Re. epend. Remnant mass of ependymal cells. 



Rect. Rectum. 



Re. int. cau. Remnant of caudal gut. 



Rud. cau. Rudiment of tail. 



Str. cell. Stria cellularis. 



Coccygeal tubercle. 



Vena sacralis media. 



Ventriculus terminalis. 



cran. Cranial portion of terminal ventricle. 



cau. Caudal portion of terminal ventricle. 

 X. Characteristic folds in ventral wall of spinal oord. 

 XX. Furrow on surface corresponding to level at which the 

 caudal gut begins to disappear. 



CO. 



Tub 

 V. s. m 

 Vent. t. 

 Vent. t. 

 Vent. t. 



Plate 2. 



Fig. 31. Embryo Xo. 810, .5..5 mm., enlarged .31. .5 diameters. The caudal gut already shows a constriction separating 

 it from the cloaca. The lines along the dorsal margin of the spinal cord represent the boundaries of the 

 myotomes. 



Fig. 32. Embryo No. 371, 6.6 mm., enlarged 31. .'i diameters. The segmental levels in this specimen are determined 

 by the sclerotomes. It will be noted that the tail has attained nearly its maximum development, and 

 as compared with the more cranial parts it will hereafter gradually take on a more atrophic appearance. 



Fig. 33. Embryo No. 389, 8 mm., enlarged 31. .5 diameters. The coccygeal portion of the spinal cord is aheady dis- 

 tinctly narrower than the main cord. 



Fig. 34. Embryo No. 544, 11 mm., enlarged 31.5 diameters. In this specimen the caudal gut has disappeared. The 

 vertebrated and non-vertebrated portions of the tail are clearly demarcated. 



Fig. 35. Embryo No. 852, 12 mm., enlarged 31.5 diameters. The non-vertebrated portion of the tail is here rela- 

 tively much shorter than in the previous specimen. 



Fig. 36. Embryo No. 390, 15.5 mm., enlarged 31.5 diameters. A rudiment of the tail persists as a small elevation 

 (rud. cau.). The caudal end of the vertebral column terminates in a fibrous strand of cells. The termi- 

 nal three sclerotomes may be regarded as having been converted into this .strand, or they may have fused 

 into one irregular vertebra. 



Pl.\te 3. 



Fig. 37. Embryo No. 406, 16 ram. crown-rump length, enlarged 31.5 diameters. The slender atrophic portion of 

 the spinal cord is clearly demarcated from the remainder of the cord owing to the fact that it retains its 

 earlier embryonic form. The point at which its narrow canal opens into the main canal corresponds to 

 the future terminal ventricle. 



Fig. 38. Embryo No. 576, 17 mm. crown-rump length, enlarged 22.5 diameters. As compared with the last specimen, 

 the caudal region has undergone marked reduction and resembles the condition that will be seen in 

 embryos about 20 mm. long. 



Fio. 39. Embryo No. 432, 18 mm. crown-rump length, enlarged 22.5 diameters. It will be noted that embryos of 

 about this size show the tendency toward a sharp dorsal retroflexion of the caudal rudiment. The charac- 

 teristic thinness and wrinkUng of the wall of the atrophic portion of the spinal cord is also well represented 

 in this embryo. 



Fig. 40. Embryo No. 453, 23 mm. crown-rump length, enlarged 22.5 diameters. In this specimen, just ventral to 

 the junction of the atrophic part with the remainder of the cord, is a mass of cells which appeared to form 

 a diverticulum, although a communication between its lumen and the central canal of the cord could not 

 be clearly made out. 



Fig. 41. Embryo 382, 23 mm. crown-rump length, enlarged 22.5 diameters. The relative narrowness of the lumen 

 of the atrophic portion of the spinal cord is a characteristic preliminary to its transition into the filum 

 terminale. Near the caudal tip, at the point marked X, is the seat of fusion with the chorda dotsalis. 



Fig. 42. Embryo No. 584a, 25 mm. crown-rump length, enlarged 22.5 diameters. A point of fusion with the chorda 

 (}orsaUs, marked X, can be seen in this specimen somewhat similar to that in figure 41. 



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