22 Dr Carpenter, oji the Development of 
examined the unfertilized ova still contained in the ovary, but 
approaching maturity, I found them to correspond in size and 
appearance with the bodies which fill the ovigerous capsules, 
but to possess the usual germinal vesicle and germinal spot. 
As it will appear, from the details to be presently given, 
that some of these bodies are time ova, whilst others are merely 
yolk-segments, it comes to be a question of much interest, 
whether the former can be distinguished from the latter, at 
this early period of their history. Neither Mr. Busk nor my- 
self, however, has been able to detect any marked difference, 
previously to the occurrence of the first segmentation. 
In the course of a few days, all the egg-like bodies in the 
capsule begin to show signs of cleavage. In the greater part 
of them, the two segments produced by the first cleavage are 
equal, or nearly so ; and each of these again subdivides into 
other two which are alike equal. The subsequent cleavages 
take place with less regularity, so that the mass often loses its 
symmetry of form ; but they commonly issue in the sub- 
division of the whole vitellus into from about twelve to eigh- 
teen segments, whose diameter averages '0030 of an inch 
(Plate III., fig. 5, a, b, c, d). 
Here and there, however, we may distinguish among the 
rest, one of these egg-like bodies whose cleavage divides it 
into two segments whose inequality is very marked (Plate III., 
fig. 6, ; and at one end of the line of constriction, there are 
to be seen one or two, or perhaps three, minute vesicles, near 
which is a clear space in each segment. These are obviously 
the ' Richtungs-blaschen,' or Vesiculce directrices, which were 
first observed by Carus, afterwards by Dumortier, Pouchet, 
Sars, Van Beneden, and Reid, and more recently by Fred. 
Miiller (who ascribes to them a great importance in the primi- 
tive development-changes of the ovum), by H. Rathke, and by 
Gegenbauer.* These vesicles were noticed by Koren and 
* It is not only in the ova of Grasteroi:>ods, that these vesicles present them- 
selves. They had heen seen in the ova of several Lamellibranchiata, Annelida, 
and Entozoa ; and in several Vertebrata also. See the Third Series of 
• Researches on Embryology ' (' Philosophical Transactions,' 1840), by Dr. 
Barry, who repeatedly figured these vesicles, but quite misapprehended 
their import ; the Memoirs of Professor Bischoff, on the ' Embryonic De- 
velopment of the Dog, Rabbit, and Guinea-pig ;' and the posthumous 
Memoir of Mr. Newport, ' On the Impregnation of the Ovum, and the 
Growth of the Embryo in the Frog,' in the ' Philosophical Transactions,' 
1854, p. 234 et seq. With reference to the idea of F. Miiller, that these 
spherical bodies determine the Kne of first cleavage of the yolk, Mr. New- 
port remarks : — " My observations on the development of the embryo lead 
me to believe that, though the transit of these bodies is usually in the 
same line as the first cleft, the direction of the fissure is not determined 
by them, but is owing to some other cause." 
