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cannot yet be said to be satisfactorily understood. It is difficult to 

 harmonize the various accounts which have been given, and all lack 

 one important feature, viz. a thorough description of the segmentation 

 processes and a tracing back of the germ-layers !to their parent cells 

 or groups of cells. In many forms difficulties which our present tech- 

 nique has not been able to satisfactorily overcome lie in the way of 

 such a study, and it was with peculiar pleasure that I discovered an 

 Isopod, Jaera albifrons. Leach, in which conditions were favorable 

 for a thorough study of a typical centrolecithal segmentation. Since 

 I wish to defer the final publication of my results until I shall have 

 had an opportunity of studying the large amount of material from 

 other species which I possess, I have thought it well to give a prelimin- 

 ary statement of the results of my study of Jaera. Owing to the cha- 

 racter of this notice references to previous observations on Isopods 

 will be omitted, and I shall content myself with a bare statement of 

 the facts I have observed. 



The e^^ oî Jaera when passed into the brood-pouch is of a bright 

 grass-green color, and is enclosed by a single envelope, the chorion: 

 it is somewhat oval in shape, and has at the centre a stellate mass of 

 protoplasm containing the nucleus, while a thin layer of peripheral 

 protoplasm encloses the yolk. A continuity of the peripheral and cen- 

 tral protoplasms cannot be distinguished clearly in mature eggs, but in 

 ovarian eggs about half grown a delicate protoplasmic network can be 

 easily seen extending between the two, the yolk granules lying in the 

 meshes. 



Two polar globules are formed at the extremity of the short axis 

 of the egg, and about the same time a second enveloping membrane, 

 the yolk membrane, is formed. The first cleavage is at right angles to 

 that of the polar globule divisions, and affects only the nucleus and 

 the central mass of protoplasm, the peripheral protoplasm and the yolk 

 remaining undivided. The second cleavage afi'ects the same parts; 

 one of the two nuclei divides in a plane at right angles to the first 

 division, while the other forms its spindle at an angle of more than 

 45° to it, and by a subsequent rotation the two nuclei so produced 

 have the line joining them at right angles to that joining the products 

 of the other nucleus. 



A somewhat similar phenomenon occurs at the third cleavage. 

 The two nuclei situated at one end of the egg divide at right angles 

 to the division of their parent nuclei, and one of the nuclei at the 

 other end divides at right angles to these two, while the fourth nucleus 

 divides at an angle of 45° to all the rest. There is thus produced at 

 one extremity of the egg (Fig. 1) a circle of four nuclei, and at the 



