226 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 Avhat 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 (PI. XI, fig. 1). The blastopore 

 narrows in a peculiar manner, one of its lips becoming reflected 

 over it (PI. XI, fig. 2), and it is finally reduced to a minute pore 

 (PI. XI, fig. 3). This opening, which is identical with the larval 

 anus, gradually travels back to near the posterior end of the 

 embryo ; this is efi"ected 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 PL XI, 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 XII, 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 to the 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 diff'erentiated into an anterior thinner- 

 walled vesicle, the coelom, and a posterior thicker-walled gut j 



