114 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



living and preserved materials. It was considered 

 essential to be able to recognize all preserved 

 stages at a later time as well as living stages at the 

 moment of collection. 



In addition to the gross morphological end- 

 points, histological examination was used on 

 occasion to define the stage more critically. The 

 criteria selected were such as to include also the 

 physiological differences. Furthermore, since the 

 status of an animal is evidenced in part tbrough its 

 activity and movements, natural movements or 

 activity were incorporated as far as possible 

 into the staging criteria. 



The method of fixation and preservation proved 

 to be suitable for recognition of practicaUj^ all 

 end-points. The normally transparent prolarvae ^ 

 became opaque in Smith's fixative. When se- 

 lected specimens were fixed and preserved in 4- 

 percent formalin, however, retention of all pig- 

 mentation and transparency aided staging. The 

 formalm, of course, hardened the yolk mass and 

 the notochord enough to prevent the use of these 

 specimens for histological preparations. 



Stage 0: Ovulated but unfertilized egg 



Animal-polo depression: Present but just 

 visible. 



Cellular areas: Nuclear; animal-hemisphere 

 cytoplasm; vegetal-hemisphere cytoplasm. 



Size: 1.0 ±0.2 millimeters. 



Ovulated eggs within the coelom are assigned 

 this stage. These creamy-white eggs are sur- 

 rounded by a relatively thin jelly coat which 

 expands when the eggs are shed into water. The 

 stickiness of this coat causes sand grains stirred 

 up by spawners to adhere to the surface of the 



egg- 



The surface of the unfertilized egg has a small 

 depression in the egg membrane over the nucleus 

 and extends into the nucleus proper. A demar- 

 cation separates the nucleus and the siu^round- 

 ing cytoplasm. One-third the distance down the 

 animal-vegetal axis the cytoplasm contains another 

 demarcation between the cytoplasm surrounding 

 the nucleus and the remaining cytoplasm. 



The egg of the sea lamprey is telolecithal in 

 that the egg consists of a relatively large amount 

 of yolk and the nucleus is located at the center 

 of the animal hemisphere. 



This stage is initiated when the eggs are ovulated 

 into the coelom and ends with fertilization. 



Stage 1: Zygote (fig. 1) hours 0-2 



Animal-pole depression: Increases in diameter 

 and depth; disappears within about 1 hour. 



Cellular areas: Identical to stage 0. 



Size: 1.0 ±0.2 millimeters. 



Fertilization membrane: Appears within 20-30 

 minutes after fertilization; is retained through 

 stage 13. 



Figure 1. — ^Lateral view of stage 1, zygote, showing 

 cellular areas. 



This stage extends from the time of fertilization 

 to the time when the fertilized egg begins to imder- 

 go first cleavage. A meridional section of a 

 fertilized egg (fig. 2) shows the yolk platelets 



' Hubbs (1943) defined a prolarva as a "larva still bearing yolk." In the 

 present work prolarva includes stages 14-17. 



Figure 2. — Meridional secUon of stage I prior to forma- 

 tion of fertilization membrane. 



