He states that the epiblast becomes divided into two layers, of which the outer is involuted into the neural cord, a median slit in the involution representing the neural groove. The eventual neural canal is stated to be lined by the involuted cells. Scott (No. 87) fully confirms Calberla on this point, and, although my own sections do not clearly shew an involution of the outer layer of epiblast cells, the testimony of these two observers must no doubt be accepted on this point. Shortly after the complete establishment of the neural cord the elongation of the embryo proceeds with great rapidity. The processes in this growth are shewn in fig. 41, A, B, and C. The cephalic portion (A, _c_) first becomes distinct, forming an anterior protuberance free from yolk. About the time it is formed the mesoblastic plates begin to be divided into somites, but the embryo is so opaque that this process can only be studied in sections. Shortly afterwards an axial lumen appears in the centre of the neural cord, in the same manner as in Teleostei. The general elongation of the embryo continues rapidly, and, as shewn in my figures, the anterior end is applied to the ventral surface of the yolk (B). With the growth of the embryo the yolk becomes entirely confined to the posterior part. This part is accordingly greatly dilated, and might easily be mistaken for the head. The position of the yolk gives to the embryo a very peculiar appearance. The apparent difference between it and the embryos of other Fishes in the position of the yolk is due in the main to the fact that the postanal portion of the tail is late in developing, and always small. As the embryo grows longer it becomes spirally coiled within the egg-shell. Before hatching the mesoblastic somites become distinctly marked (C). The hatching takes place at between 13-21 days after impregnation; the period varying according to the temperature. [FIG. 41. FOUR STAGES IN THE DEVELOPMENT OF PETROMYZON. (After Owsjannikoff.) _c._ cephalic extremity; _bl._ blastopore; _op._ optic vesicle; _au.v._ auditory vesicle; _br.c._ branchial clefts.]