J34 A - RICHARDS AND JAMES T. THOMPSON. 



portation but cell division takes part. Certain workers with other 

 forms have held that the movement of the cells into the anlagen 

 of the gonads is not the only factor responsible for their increase, 

 but that multiplication actually occurs during the period of trans- 

 location. Mitotic figures have never been observed in Fundulus 

 among the recognizable sex-cells which are within the embryo, 

 although a most thorough search has been made for them in many 

 embryos at all stages of development. A count of these cells in 

 several specimens in various stages reveals the fact that there is a 

 tendency for their number to vary more or less from the average 

 established (67). However there is not enough variation to con- 

 vince one that there is any marked multiplication of the sex-cells 

 during the migration period. These facts naturally lead to the 

 conclusion that the first period of multiplication takes place in 

 the extra-embryonic region. 



In the description of the earliest embryos referred to in this 

 report and in the figures presented, emphasis has been placed 

 upon the fact that the primary sex-cells in any one embryo are 

 not in the same phase of migration. Furthermore observations 

 upon all stages show that development becomes more advanced 

 anteriorly than posteriorly. It is of further interest that in em- 

 bryos containing sex-cells both within and without the body, the 

 number falls below the average for older stages. These condi- 

 tions and the fact that no sex-cells have been found in any region 

 other than that already described, suggest the explanation that 

 these cells multiply in the extra-embryonic region. Indeed the 

 four sex-cells illustrated in Fig. 12 may indicate recent cell divi- 

 sion by their very association. If they are not of recent and iden- 

 tical origin they would probably be farther separated than they 

 are in this figure. These views are presented only tentatively, 

 due to lack of sufficient material to warrant definite statements 

 on this multiplication ; for no mitotic figures have ever been seen 

 in the extra-embryonic region to substantiate this belief in a divi- 

 sion as suggested. Our material has not permitted a careful study 

 of this matter. However considering the longitudinal distribu- 

 tion of the sex-cells in the earliest available embryos, and their 

 tendency to approach a common average number in each indi- 

 vidual, one is inclined to regard them as being of unquestionably 



