60 SUMMABY OF CUKRENT RESEARCHES RELATING TO 



and are, at the time they are laid, distinguishable only by their size 

 and colour. They undergo segmentation in the «ame way. At a 

 certain time, the embryo is encysted in a cellular membrane lying 

 internally to the vitelline membrane. Later on this becomes chitinized 

 and ornamented, while the outer vitelline membrane disappears. 



12. As far as the closure of the blastopore the male egg has the 

 same history as the female. 



13. The summer egg, if not fecundated, does not give off polar 

 globules. 



14. When the egg is laid it divides into two unequal segments, 

 which divide regularly and symmetically up to the 16th stage. After 

 this the derivates of the small segment predominate and inclose the 

 others. When the blastoderm is formed the embryo consists of — 



a. An internal layer entirely derived from the last and largest 

 segmentation-sphere ; this forms the intestine. 



6. An outer layer which forms the ectoderm and is in great part, 

 probably altogether, derived from the smaller primitive segment, and 

 from the first sphere detached from the large segment. 



c. A median layer which, if it does not form a continuous layer, 

 at least does form groups of cells which are arranged between the 

 outer and inner layers ; this is derived from the two median spheres 

 of the large primitive segment, and probably serves to form the 

 genital organs and muscles. 



This arrangement is a striking example of the way in which the 

 order of the succession of the layers corresponds to the order of 

 segmentation, the sphere the furthest from the animal pole serving 

 exclusively to form the intestine, the two median spheres the genital 

 organs and muscles, and, lastly, the three lower clear spheres the 

 ectoderm. 



15. When the blastoderm has been formed the embryological 

 phenomena take place in the following order : formation of the 

 tail ; appearance of the vibratile pit, of the short cilia that cover it ; 

 development of the ocular pigment, of the large cilia of the wheel- 

 organ ; formation of the buccal cavity and of the cloaca by the in- 

 vagination of the ectoderm ; appearance of the meconium, of the 

 mastax, of the vibratile cilia. The larva then escapes, leads for 

 some time a wandering life, and then settles down to construct its 

 tube. 



Echinodermata. 



Histology of Echinodermata.* — In his second communication f 

 O. Hamann commences with an account of the nervous system of 

 Holothuria polii, and of the structure of the " feet " of this form. In 

 each of the so-called pyramidal feet we find a strong nerve-cord 

 placed in the connective tissue, and composed of epithelial supporting 

 cells, between the processes of which run the nerve-fibrils ; below the 

 apical disk they form a plate, and here the processes of the epithelial 

 sensory cells pass into the layer of nerve-fibrils. Among the nerve- 



* Zeitschr. f. Wiss. ZooL, xxxix. (1883) pp. 309-33 (3 pis.), 

 t See this Jourual, iii (1883) p. 847. 



