CYCLOSTOMATA, 



69 



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

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

 primitive Cyclostomata had not tiue 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 

 prsegnathostomatous 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 gnathostomatous Vertebrata. 



The ripe ovum of Petrorayzon Planeri is a slightly oval body of 

 about 1 mm. in diameter. It is mainly formed of an opaque nearly 

 white yolk, invested by a membrane composed of an inner perforated 

 layer, and an outer structureless layer. There appears to be a 

 pore perforating 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 sucto- 

 rial 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 \ 



The segmentation is total and unequal, and closely resembles 

 that in the Frog's egg (Vol. i. p. 78). The upper pole is very slightly 

 whiter than the lower. A 

 segmentation cavity is formed 

 very early, and is placed be- 

 tween the small cells of the 

 upper pole and the large cells 

 of the lower pole. It is pro- 

 portionately larger than in the -, ; 

 Frog ; and the roof eventually 

 thins out so as to be formed 

 of a single row of small cells. 

 At the sides of the segmenta- 

 tion cavity there are always 

 several rows of small cells, 

 which gradually merge into 

 the larger cells of the lower 

 pole of the egg. The segmen- 

 tation is completed in about 

 fifty hours. 



The segmentation is fol- 



TnC. 



y 



Fig. 37. Longitudinal vertical section 



THROUGH AN EMBRYO OF PeTROMYZON PlANERI 

 OF 136 HOURS. 



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

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

 cavity. 



^ Artificial impregnation may be effected witiiout difficulty by squeezing out into tlie 

 same vessel the ova and si)erniatozoa of a ripe female and male. The fertilized eggs 

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



