12 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



After 12 hours of development the arc of the blastoderm occupies 

 about one-fifth of the meridional circumference of the egg. Its inner 

 surface is nearly flat, the outer slightly convex and the margins thin. 

 In its central part the cells are 3 to 4 layers deep and, owing to the 

 repeated horizontal divisions, now smaller than the marginal. The 

 latter are surrounded by a wide periblastic wreath containing numerous 

 nuclei. 



At 16 hours the blastodermic rim is inflected and the embryonic 

 thickening begins to appear. 



At 20 hours the blastoderm occupies about 35 per cent of the surface 

 of the ovum, and the embryonic shield has extended to its vertex. 



At 30 hours the blastoderm has covered about 50 per cent of the 

 surface of the ovum and the body of the embryo is distinctly outlined. 



At 40 hours the blastopore is still about 0.5 mm. in diameter, and the 

 embryo extends around about 35 per cent of the egg circumference. 

 The notochord is distinctly differentiated. The eyes are just becoming 

 defined as out-pushings. The embryonic body is deep and compressed 

 below, but no myotomes are visible. No pigment has yet developed. 



At 45 hours the blastopore has decreased in diameter to an average of 

 0.4 mm. The embryo extends nearly halfway around the egg. The eyes 

 stand out from the head quite prominently as rounded masses. The first 

 myotome is formed. 



At 52 hours the embryo measures in total length 1.7 mm.; 5 myo- 

 tomes, of which the first 2 are very distinct, may be counted in the 

 middle region of the trunk. The head is clearly marked off. Pigment 

 begins to appear and the chromatophores are arranged along tbe mar- 

 gins of the trunk of the embryo for its entire length. The nervous 

 axis is very conspicuous. 



At 60 hours the length of the embryo has increased to 1.9 mm. Pig- 

 ment has become much more abundant and is aggregating on the 

 head. There are 11 distinct myotomes. The crystalline lens is 

 appearing as a thickened in-pushing of the epiblast. The blastopore 

 is nearly closed. 



At 70 hours the length of the extended embryo is 2 mm. The epi- 

 blastic involutions for the crystalline lens and the ear-sac are distinctly 

 marked. The nasal involution is beginning to appear. The fourth 

 ventricle of the brain is differentiated and the cerebellum is folding off. 

 The lateial-line sense organs are apparent as epiblastic thickenings. 

 There are 19 myotomes. The pigment cells have become more con- 

 spicuous and branching. No pigment is present anterior to the eyes. 

 A small pointed caudal tubercle is forming as a preliminary to the 

 outgrowth of the tail. 



At 78 hours the embryo measures 2.1 mm.; 20 or 21 myotomes are 

 present. The ear-sacs are now closed vesicles, just anterior to which 

 appear the beginnings of a large sense-organ. The lens has sunken 

 deep into the optic cup, its inner wall is thickened, and its orifice is 

 closing. The olfactory vesicles also have their orifices much constricted. 



