No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 95 



as in Jaei-a, the Mittelstrang. Behind the teloblasts is a mass 

 of ectoderm cells more or less irregularly arranged, which rep- 

 resent the hind end of the body, and among which the anus 

 will form. 



With regard to the naupliar portion of the embryo at this 

 stage, there are but few points to be noted. The separation 

 between the naupliar and post-naupliar regions is clearly indi- 

 cated by the scattered arrangement of the cells, and the lateral 

 ventral bands have approximated still further than in Fig. 37. 

 On each side, just at the junction of the two regions, is to be 

 seen a small patch of cells which constitutes the Anlage of the 

 lobed (lappen-formige) organ of Aselliis. 



3. TJie Segmentation and Fonnation of the Get-m-layers m 

 Porcellio and Arinadillidiwn. 



The similarity between the ova of Porcellio and Arniadilli- 

 diiim in their development is so great that the two forms may 

 be considered together. 



Porcellio has been the subject of several papers, which pre- 

 sent most remarkable discrepancies in the descriptions given 

 of the segmentation, and, in considering them, papers on 

 Onisciis may also be included, since my own observations on 

 forms belonging to this genus and on the nearly related genus 

 Philoscia, though most fragmentary, nevertheless clearly show 

 that the type of segmentation is identical with that of Porcellio. 

 The earliest paper is by Bobretzsky ('74) on Oniscns nmrariiis, 

 and the results recorded in it have been accepted and copied 

 into many general works. According to Bobretzky the seg- 

 mentation of Onisciis is of the epibolic variety, and resembles 

 that described by Van Beneden ('69) for Mysis. According to 

 his account a quantity of protoplasm collects at one pole of 

 the tz%y forming a colorless, transparent, rounded mass, which, 

 in the next stage observed, is represented by a plate or disc of 

 cells lying sometimes at the pole of the &^^, sometimes near 

 the pole, and sometimes upon the side. In later stages this 

 disc extends over the yolk, the mes-endoderm forming from it 

 before it reaches the equator in cases where it originally occu- 



