128 



FISHERY BULLETIN OF THE FISH AND WTLDLIFE SERVICE 



REVIEW OF STAGES AND COMPARISON 

 WITH EARLIER STUDIES 



Staging of lampreys in most early literature 

 lacked precision. Investigators, for example, 

 Shipley 1885, Scott 1887, McClure 1893, Hatta 

 1914, 1915, who studied early embryology, gave 

 little or no attention to the need for staging. 

 Developmental sequence was described in con- 

 ventional embryological terms. 



The first serious attempt to show stages of 

 lamprey development was by Damas (1944) in his 

 excellent histological study of L. fluviatilis, con- 

 cerned primarily with tracing development of the 

 head. Each section (comparable to a stage) com- 

 prised a short account of external features and a 

 thorough histological description. His staging 

 series began with embryos possessing 3 somites 

 (section 1). The next section included embryos 

 with 10 somites. (Every somite listed by Damas 

 was not visible in external view, but each was seen 

 histologically.) On the basis of somite numbers 

 Damas was able to describe several stages slightly 

 different from each other which could be described 

 also by staging based on external morphological 

 features. Some of his sections, based usually on 

 intervals of 10 somites, have been translated into 

 stages of the present study for comparison in 

 table 1. 



Stages described in the present work have been 

 based primarily on changes of external features. 

 Histological observations have been used sparingly 

 and merely for clarification. Damas, who began 

 his study after development had started, omitted 

 the first 9 stages of the present work. Because 

 his criteria were histological, his stages are over- 

 lapped by stages in the present study (table 1). 



Table 1. — Equaling o/ "sections" of Damas (1944) study 

 with "stages" of the present research 



Damas (section), 

 Plavis (stage) . . . 

 Damas (section) 

 Piavts (stage) . . . 

 Damas (section) 

 Plavis (stage) . . . 



V 

 13 

 X 

 14 

 XV 

 18 



Staging, to be of practical value, should be 

 simply, easily, and promptly recognized by inves- 

 tigators in both living and preserved materials. 

 By far the easiest method of designating stages 

 is one based on morphological characteristics 

 since the resulting demarcations are relatively 

 sharp and distinct and the diagnostic features are 



visible without elaborate histological preparation. 

 They are natural divisions in a developmental 

 sequence. Particular effort was made in this 

 study to avoid dependence on measurements and 

 counts. Ideally, observation alone should suffice 

 for recognition of stages. 



Applegate (1950) divided the larval forms of 

 P. marinus on the basis of length and weight into 

 age groups, beginning with age-group 0. He 

 stated that the larvae "... upon leaving the 

 nest have completed their early developmental 

 stages and are perfectly formed but diminutive 

 ammocoetes." These ammocoetes are in reality 

 stage 18 larvae, for specimens from Applegate's 

 collections match specimens from my collections 

 in their external as well as histologic characteristics. 

 Stages, as designated here, can be determined 

 by the naked eye or with a hand lens. Separation 

 of stages 12 and 13 might conceivably cause some 

 difficulty since transition between them is based 

 on muscular activity. It is easy, however, to 

 recognize preserved specimens, for the head region 

 of embryos of stage 13 is displaced to either side 

 of midhne whereas heads of stage 12 embryos are 

 medial. 



Cleavage in the sea lamprey has several interest- 

 ing characteristics, the first of which is the appear- 

 ance of relatively high prominences lateral to and 

 above the first cleavage furrow. Similar promi- 

 nences occur during second cleavage (stage 3). 



McClure (1893) noted the unusual cleavage 

 pattern of P. marinus, especially in the third and 

 fourth cleavages. He described tlie third cleavage 

 as meridional and considered this to be the sole 

 type of third cleavage. Actually, both meridional 

 and equatorial types of third cleavage occur, but 

 their relative abundance fluctuated widely irre- 

 spective of temperature. 



Teleostei, Gymnophiona, Gallus, and other 

 forms exhibit a meridional third cleavage, vvhich, 

 however, is accomplished in mcroblastic division. 

 In the sea lamprey the meridional third cleavage 

 is holoblastic. 



The cleavage of stage 4 embryos determines the 

 type of cleavage for stage 5, since an equatorial 

 stage 4 is followed by meridional stage 5 and 

 vice versa. 



Cleavage of stage 6 embryos is indeterminable 

 and is accompanied by a lag in the cleavage of 

 the vegetal cells. After stage 6, demarcation be- 

 tween the animal and vegetal cells is sharp until 



