64 MCMURRICH. [Vol. XI. 



servation. A preliminary notice of my observations upon this 

 form was published in the Zoologischer Anseiger in the summer 

 of 1 892, and seemed of sufficient interest to lead me to extend 

 my studies to Ascllus communis, Say, and later to Porcellio 

 scabcr and Armadiliidium vnlgare, the greater portion of the 

 material being collected at Woods Holl. To my friend, 

 Dr. W. M. Wheeler, I am indebted for a considerable quantity 

 of material, which came originally, I believe, from Naples, and 

 illustrates the development of CymotJioa, and to Dr. F. H. 

 Herrick for material from Ligia, collected at Beaufort, N. C. 

 In neither of these cases, however, were the earlier stages of 

 segmentation represented, and I could make use of them only 

 for the later stages of development. 



In the present paper I do not propose to enter in detail into 

 the organogeny of the group, my observations on this depart- 

 ment of the subject not yet being sufficiently advanced. I 

 shall have occasion, however, to refer to certain facts which 

 have been made out concerning the development of some of the 

 organs, and consequently shall divide the paper into four por- 

 tions, the first of which will treat of certain phenomena con- 

 nected with the impregnation and formation of the egg-mem- 

 branes of Jaera ; the second, of the segmentation and the 

 development of the germ-layers ; the third, of the later devel- 

 opment of the mesoderm and endoderm ; while in the fourth, 

 some scattered notes concerning the development of certain of 

 the organs will be discussed, 



A word is necessary with regard to the identification of 

 Jaera marina. In my preliminary notice ('92) I referred to it 

 z& Jaera albifrons, on the authority of Harger ('78). In a more 

 recent paper by Sye ('87) it is stated that the form described 

 by Fabricius in his Fauna Groenlandica as Oniscus mariniis is 

 identical with that later described by Montagu as Oniscus 

 albifrons. In 181 5 Leach united these two forms in the genus 

 Jaera, and adopted for the species the name of /. albifrons, 

 which is the authority for the name employed by Harger. If, 

 however, the two species are identical, then Fabricius's specific 

 name has the priority, and the entire name should be properly 

 that employed by Mobius in 1873, Jaera marina. As regards 



