ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 61 



ectoderm gradually grows round the embryo, but the pharynx remains 

 closed until the time of hatching. 



Development of Planarians.* — Mr. W. G. Van Name has obtained 

 numerous eggs of Eustylochus ellipticus and Planocera nebulosa, and has 

 been able to make a series of observations on their maturation, ferti- 

 lisation, and early development. In Eustylochus the sperm-centrosome 

 and sperm-aster could not be made out, but otherwise the observations 

 ir.ade on the two forms were practically identical. 



The germinal vesicle is large, and contains a nucleolus and a network 

 of chromatin-threads. The entrance of the spermatozoon into the egg 

 occurs at the time when the first polar spindle begins to form. The two 

 centrosomes appear at once, but their exact point of origin was not 

 made out. They are surrounded each by a centrosphere which at first 

 has only few rays. These early stages take place before the egg is 

 laid. Usually also, before it is laid, the centrosomes have divided. In 

 the formation of the first polar body a true case of heterotypical mitosis 

 (Flemming) occurs. After the extrusion of the first polar body, the 

 remains of the old centrosphere surrounding the separating centrosomes 

 rounds itself off in each case, and forms a new centrosphere about each, 

 so that the entire mitotic figure of the second polar body, with the ex- 

 ception of the chromosomes and part of the aster rays, is formed from 

 the centrosphere of the inner pole of the preceding spindle. The outer 

 part of the aster-rays appears to arise from the granules of the cytoplasm. 

 The chromosomes of the second polar spindle are much smaller than 

 the first. It is probable, but not certain, that their division is a trans- 

 verse and reducing one. After the extrusion of the second polar body, 

 the aster degenerates, and centrosphere and ceutrosome disappear. 

 Though the spermatozoon penetrates the egg while it remains in the 

 uterus, fertilisation in the strict sense does not occur till the egg is laid. 

 In Planocera the sperm-centrosome is not distinct till the late stages 

 of the first polar spindle, and is then surrounded by short rays made 

 up of microsomes. Later it divides into two. The relation of these 

 sperm-centrosomes to the cleavage-centrosomes was not demonstrated, 

 nor could the origin of the cleavage-centrosomes be made out in either 

 Planocera or Eustylochus. Similarly, it was found impossible to trace 

 the relation between the centrosomes of successive cleavages, or to work 

 out the cell-lineage. 



Development of Convoluta.j — Prof. J. Georgevitch has studied the 

 early stages in the development of Gonvoluta roscoffensis. Maturation 

 and fertilisation occur within the body ; each ovum has a transparent 

 capsule, within which the development may be seen in progress on to 

 the gastrula stage. The first cleavage forms two almost equal blasto- 

 meres ; then two smaller cells are formed ; a minute segmentation cavity, 

 present at the 4-cell stage, disappears after the 8-cell stage. The two 

 small blastomeres (ectoderm) indicate the dorsal portion of the future 

 animal ; the two larger blastomeres (endoderm) indicate the ventral 

 side. After a brief quiescence the endoderm cells divide laterally and 

 form the initial cells of the mesoderm ; the two ectoderm-cells also 

 divide, and an 8-celled blastula results with marked bilateral symmetry. 



* Trans. Connecticut Acad., x. (1899) pp. 263-300 (5 pis.), 

 t Arch. Zool. Expe'r., vii. (1899) pp. 343-61 (1 pi.). 



