506 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



is doubtful. Bischoff only made the observation in one animal, and 

 since that period nothing has been ascertained in regard to the occur- 

 rence of rotation in the unsegmented ovum.* 



SEGMENTATION AND THE FORMATION OF LAMINA. 



(a.) In Batrachia. The segmentation of the ova in Batra- 

 chia was discovered in the year 1824, by Prevost and Dumas,f 

 and given in full detail by Rusconi,: in 1826. 



No material is so favourable as the ova of Batrachia for 

 the observation of this process. With the return of the 

 first days of spring the spawn may be obtained in large 

 quantities ; and if the ova of various species are investi- 

 gated, they may be repeatedly examined at intervals varying 

 from a few days to several weeks. 



Another advantage is that the cleavage proceeds under 

 our eyes, without the necessity of any interference on our 

 part. It is only requisite to place a spawn chain (Bufo) 

 or a mass of spawn (Rana) in water in a watch glass, and 

 with the aid of a lens the whole process may be conveni- 

 ently followed. The vitelline membrane is not visible (by 

 direct light) by this mode of examination, and we obtain the 

 impression that the germ itself undergoes cleavage upon its 

 surface. If, however, we place a group of ova in a watch 

 glass, and examine them by direct light with somewhat higher 

 powers (as from forty to fifty linear) we may soon convince 

 ourselves that the transparent membrane takes no part in the 

 cleavage. 



The formation of the first grooves in the Batrachian ovum 

 may best be rendered intelligible by taking a ball of modelling 

 clay, and constricting it in the following manner. Place a thread 

 in one meridian, and a second at right angles to it. Tighten 

 the two threads so that the upper third of the spheroid is cut 

 through by them. A third thread may now be placed parallel 



* Entwickelungsgeschichte des Kaninchens, (History of the development 

 of the Rabbit,) 1842. At pp. 58 and 59 of this work the reader will find 

 the literature bearing upon the rotation of the yolk. 



t Annales des Sciences, S^r. i., Tom. ii. 



t Developpement de la Grenouille. 



