DEVELOPMENT OF HALISARCA LOBULAEIS. 605 



duced phenomena. Some minute points of histological detail 

 may, however, be regarded as results of treatment. 



The ovum (PI. XXXVII, fig. 1), which F. E. Schulze has traced 

 from a wandering mesoderm cell, exhibits when 0.04 mm. in dia- 

 meter a distinct external layer of structureless material, which 

 takes a deep stain with reagents^ and which appears to remain in 

 organic connection with the surrounding matrix of connective 

 jelly by means of short pseudopodia-like processes. Within the 

 outer layer is a finely granular protoplasm, enclosing an excentri- 

 cally placed spherical nucleus O'Ol mm. in diameter, within which 

 again, is a spherical, transparent, highly refringent nucleolus, 

 0-0045 mm. in diameter. It stains deeply, and is surrounded 

 by finely granular, scarcely-stained material, which nearly fills 

 the sharply-contoured vesicle of the nucleus. This no doubt 

 results from the coagulation of the liquid contents of the 

 nucleus. 



The most external layer of the cell is said by Schulze to be 

 invisible in living specimens, and he regards it as an artificial 

 product. No doubt its remarkably clear difi'erentiation from 

 the rest of the ovum is due to technical treatment ; but 

 this treatment merely brings it to light. If it existed as a 

 separate layer in the living ovum it would be difficult to dis- 

 tinguish from the internal plasma, unless it difi'ered greatly in 

 refractive index. I am inclined to regard it as the rudiment 

 of a blastema which we shall afterwards find existing as a most 

 noticeable part of the segmented ovum. 



The segmentation of the ovum, already known to be entire, 

 proceeds with great irregularity. Though the stages of 2 (fig. 2) 

 and 4 (fig. 15) are frequently in every respect normal yet just as 

 often they present curious irregularities. In the stage 2, we 

 sometimes find one comparatively very large, and one very small 

 segment. A stage of three segments sometimes occurs, some- 

 times explicable as due to retarded division of a large segment 

 (fig. 3), sometimes not. Stage 4 is usually normal, but one 

 curious form is shown in fig. 4^ where two opposite, not ad- 

 jacent, segments remained united by a narrow bridge of proto- 

 plasm. A stage shown in section as five is not unusual, the 



