FORMATION OF THE LAYERS. 



descended from a relatively highly-organized type of Fish. It appears to 

 me almost certain that they belong to a group of fishes in which a true 

 skeleton of branchial bars had not become developed, the branchial skeleton 

 they possess being simply an extra-branchial system ; while I see no reason 

 to suppose that a true branchial skeleton has disappeared. If the primitive 

 Cyclostomata had not true branchial bars, they could not have had jaws, 

 because jaws are essentially developed from the mandibular branchial bar. 

 These considerations, which are supported by numerous other features of 

 their anatomy, such as the character of the axial skeleton, the straightness 

 of the intestinal tube, the presence of a subintestinal vein etc., all tend to 

 prove that these fishes are remnants of a primitive and prasgnathostomatous 

 group. The few surviving members of the group have probably owed 

 their preservation to their parasitic or semiparasitic habits, while the 

 group as a whole probably disappeared on the appearance of gnathostoma- 

 tous Vertebrata. 



The ripe ovum of Petromyzon Planeri is a slightly oval body 

 of about i mm. in diameter. 

 It is mainly formed of an 

 opaque nearly white yolk, 

 invested by a membrane 

 composed of an inner per- 

 forated layer, and an outer 

 structureless layer. There 

 appears to be a pore per- 

 forating the inner layer at 

 the formative pole, which 

 may be called a micropyle 

 (Kupffer and Benecke, No. 

 79). Enclosing the egg- 

 membranes there is present 

 a mucous envelope, which 

 causes the egg, when laid, 

 to adhere to stones or other objects. 



Impregnation is effected by the male attaching itself by its 

 suctorial mouth to the female. The attached couple then shake 

 together ; and, as they do so, they respectively emit from their 

 abdominal pores ova and spermatozoa which pass into a hole 

 previously made 1 . 



1 Artificial impregnation may be effected without difficulty by squeezing out into 

 the same vessel the ova and spermatozoa of a ripe female and male. The fertilized 

 eggs are easily reared. Petromyzon Planeri breeds during the second half of April. 



FIG. 37. LONGITUDINAL VERTICAL SEC- 

 TION THROUGH AN EMBRYO OF PETROMYZON 



PLANERI OF 136 HOURS. 



we. mesoblast ; yk. yolk-cells ; al. alimen- 

 tary tract; bl. blastopore ; s.c. segmentation 

 cavity. 



