492 SUMMARY OF CURRENT RESEARCHES RELATING TO 



nor less than the segmentation cavity. The space surrounding the heart 

 is a portion of the true coelom. The heart is produced by the formation 

 of the central mesoblastic cells into a tube which, as soon as it has a 

 lumen, communicates with the space between the ventral epiblastic body- 

 wall and the periblast. The cavity in which it is contained is due 

 to a splitting of the mesoblast. 



The interesting morphological peculiarity about the venous sinus 

 in the Teleostean embryo is that it is the persistent segmentation cavity. 

 This may partially disappear owing to the contact of its walls, but it is 

 not obliterated by the growth of the mesoblast, so that, when the sinus 

 venosus appears, it is not as a cavity or system of veins entirely sur- 

 rounded by splanchnic mesoblast, but is the old segmentation cavity 

 between the epiblastic ventral wall of the yolk-sac and the periblast. 

 At a later stage, no doubt, the sinus venosus acquires mesoblastic walls 

 all round it, but this is not till the yolk has been absorbed. 



/3. Histology* 



Vital Processes in Living Cells. f — Prof. C. Frommann has made a 

 study of the vital processes in living cells. He commences with an 

 account of ripe unfertilized and fertilized ova of Strongylocentrotus lividus. 

 The granules found in the protoplasm are connected partly by very fine 

 and partly by somewhat coarse and short filaments. The processes 

 between the granules lead to the formation of extremely fine or some- 

 what ,:.oarser plexuses. The radiate marking which is seen in the 

 periphery of some eggs is due partly to rather long fine filaments, 

 which are beset with separate granules or with small spindle-shaped 

 nodules, and partly by somewhat coarser indistinctly granulated cords 

 which are connected by processes with their neighbours ; they sometimes 

 take a zigzag course. 



All the formed parts of the egg undergo a constant change of form 

 and size as well as some alteration in their refractive power ; they fuse 

 with one another or divide into two or more fragments ; they disappear, 

 while others are freshly formed ; and all these processes occur so 

 rapidly that it is quite impossible to figure all the successive images that 

 are presented. These alterations may, moreover, occur in the most varied 

 manner. Coarser filaments may break up into distinct granules which 

 separate from or unite with one another; others become indistinctly 

 granulated or disappear altogether. These processes are often preceded 

 by a division of the filaments into two or more pieces, which may under- 

 go various kinds of changes. Changes in form may accompany alterations 

 in the characters of the filaments, and they may become bent, hoop- 

 shaped, or united by bonds with their neighbours. Coarser granules 

 exhibit corresponding structures. 



The same kind of changes in the yolk-substance are seen in fertilized 

 as well as in unfertilized eggs, and there can be no doubt that, so far as 

 its vital changes are concerned, the yolk-mass completely corresponds to 

 the protoplasm of other cells. 



The rounded or oval homogeneous egg-nucleus has a boundary which 

 alters in character ; there is often a delicate, pale, and unbroken 

 contour which may yield to one which is delicately granular or 



* This seel ion is limited to papers relating to Cells and Fibres. 



t Jenaische Zeitsclir. f. Naturwiss , xxiii. (1889) pp. 389-412 (1 pi.). 



