344, E. W. MACBRIDE. 
is not quite spherical, its long axis exceeding very slightly its 
transverse axis, so that we can see that the blastopore is 
situated in the centre of what afterwards becomes the ventral 
surface. The gastrula has acquired a uniform covering of cilia, 
and the blastopore is a round opening with well-defined lips. 
This well-marked stage of development, which is easy to 
recognise, I have called Stage A (P1.18, fig.1). The blastopore 
narrows in a peculiar manner, one of its lips becoming reflected 
over it (Pl. 18, fig. 2), and it is finally reduced to a minute pore 
(P1. 18, fig. 3). This opening, which is identical with the larval 
anus, gradually travels back to near the posterior end of the 
embryo; this is effected by differences in the rate of growth of 
surrounding parts. During this time the embryo has been 
lengthening its long axis, and on the fourth day it ruptures the 
vitelline membrane and escapes. It then has the form shown 
in P1. 18, figs. 4—6, and as this stage is also a well-marked one, 
I have called it Stage B. 
The foregoing is Ludwig’s account ; my material was not 
suitable for such observations, which ought to be made on the 
living embryos, and I had not the opportunity of observing 
these early stages alive. As far, however, as I could make out, 
Ludwig is perfectly correct in his statements. I was able to 
recognise Stage A, for instance, with ease. 
Let us turn now to the internal changes which have gone on 
during this time. Pl. 19, figs. 20 and 21, are two sections of 
an embryo of Stage A, and they form the starting-point of the 
changes we shall have to consider ; I may here say at once that 
all sections which illustrate the development of the larva and 
its metamorphosis are to be understood to have been cut 
parallel tothe larval plane except the contrary is dis- 
tinctly affirmed. Fig. 22 is a sagittal section of a slightly 
older embryo ; here mesenchyme cells have appeared. The 
large size of the archenteron is a remarkable feature, the 
blastocoele or segmentation cavity, usually spacious in Echino- 
derms, being reduced to a mere slit. Fig. 23 shows us that the 
archenteron becomes differentiated into an anterior thinner- 
walled vesicle, the coelom, and a posterior thicker-walled gut ; 
