The germ-cells. 62? 



oüly be described as very abnormal positions. For this reason there 

 seems to be a decided advantage in beginning the description at a 

 well advanced phase, such as this; for in this embryo no embryologist 

 would have the slightest hesitation as to the germ-nature of a single 

 one of the cells to be described and recorded as germ-cells ^). 



As characteristic of embryos of this period — the degree of de- 

 velopment of the organs of No. 454 not being recorded in my notes 

 — the description of another embryo (No. 209 = 34 mm) may be 

 given. The long tail of embryos of R. hatis accounts for so much of 

 the growth, that the additional 2 mm are quite insignificant. 



Externally. There are well developed external gills on the 

 five branchial arches. The longest do not exceed 2 mm. The upper 

 jaws are beginning to be formed, the mouth is widely open. Pectoral 

 and pelvic fins are well-marked, and the two unpaired dorsals are 

 present. 



Internally. The sensory foundations of the dermal sense- 

 organs are deploying above the eye. There is an olfactory pit, but it 

 is widely open. In the eye the retinal layer is much thicker than 

 the future pigmented one. The optic stalk is still widely open, and 

 there is as yet no trace of an optic nerve. There is no pigment in 

 the retina. Lens-fibres are forming. The head -somites show no signs 

 of muscle-formation. The auditory organ is a simple sac, and gangli- 

 onic proliferation from its epithelium has ceased. The lateral line 

 reaches back to the beginning of the genital organs. Histological 

 differentiation is beginning in the medulla. There is a marked marginal 

 veil in the spinal cord. In the occipital region there is pro-cartilage 

 on each side of the notochord. The thymus-placodes are only slightly 

 thickened. The oesophagus is occluded. The pancreas is in an early 

 phase. The spiral valve does not project much into the lumen of 

 the gut. The rectal gland shows much budding. The lumen of the 

 rectum is occluded. In section the pelvic fins are not large, and 

 muscle-buds are just entering them. The sexual organs, i. e., the 

 "germinal ridges", project slightly (compare Fig. 37). The segmental 

 ducts extend to the cloaca, where their ends are fused together and 

 with the cloacal epithelium. The mesonephric tubules open into the 

 segmental duct. The cloaca is closed. 



1) Even years of intimate acquaintance with particular forms of 

 cells do not protect the investigator from the criticism and reproach 

 of ignorance! (Vide: Ergebn. Anat. Entw., V. 6, p. 710, 1897.) 



