344 iviLSoy. [Vol. ix. 



the stage of four. In these early stages the nuclear membrane 

 could not be made out, the clear space round the nucleolus 

 being only vaguely outlined. In a later stage, Fig. io8, prob- 

 ably sixteen segments, the membrane first makes its appear- 

 ance, though it has no doubt been there the whole time. An 

 advanced stage of segmentation is shown in Fig. 109, a still 

 later stage in Fig. no, an embryo as yet unciliated in Fig. 

 Ill, and the ciliated free swimming larva in Fig. 1 12. 



In F"ig. 1 10 is shown a curious phenomenon. What appear 

 to be cell membranes are distinctly seen round many of the 

 segments, the body of the cell in some cases having fallen out 

 of its surrounding membrane. These membranes are proto- 

 plasmic and are undoubtedly artefacts caused by the fixing 

 fluid. They are of interest only as indicating how sharp the 

 demarcation is between the cortical layer of pure protoplasm 

 which invests each segment, and the yolk-containing protoplasm 

 which makes up the mass of the segment. In the sudden con- 

 traction due to the stimulus of the fixing fluid, it would appear 

 that the central and cortical protoplasms of one segment part 

 company more easily than the cortical layers of adjacent seg- 

 ments, which in life must be closely appressed. Though I 

 regard these "membranes" as artefacts, I am aware that 

 Schulze has described somewhat similar structures in the larva 

 of Euspongia (37), which he considers to be of a normal nature. 

 Between the parenchyma cells of the solid larva of Euspongia, 

 Schulze describes partitions, which he is in doubt whether to 

 regard as secretions or as the modified cortical layers of the 

 cells. He thinks it probable that they are later transformed 

 into the uniform watery jelly in which the cells lie, and points 

 out the analogy to cartilage, comparing the intercellular par- 

 titions with the capsules which go to form the cartilage matrix. 



The cells of the embryo, Fig. in, are full of fine yolk, and, 

 being very closely appressed, the outlines are indistinct as 

 compared with earlier stages. The metamorphosis of the large 

 yolk spheres of the ripe egg into fine yolk goes on gradually 

 during the segmentation (compare Figs. 104, 106, 108, 109, 

 1 10, III), retracing the path which was followed in the devel- 

 opment of the small egg cell into the ripe ovum. 



