No. 465.] STUDIES OF MYXINOIDS. 661 
c had a cap reaching a little more than a millimeter past the 
ring, and /, in which I had not previously noticed any terraces, 
had a cap a trifle deeper than thatof c. The caps in the earlier 
stages are very hard to detect, as the difference in translucency 
between them and the rest of the egg is very slight, and the 
shell somewhat opaque. 
Wednesday morning at the end of the first week, a’s cap was 
13 mm. deep, /'s 9 mm, cs 7 mm., f’s 8 mm., while æ, which had 
shown terraces almost as soon as c, lagged behind with a cap of 
. only a little more than 41 mm. 
By five P. M., a’s had grown 1 mm, 5 had not changed at all, 
while on the others the caps had grown 2 mm. each, twice the 
usual amount. That night, however, they all grew at the regu- 
lar rate. Meantime, the white speck on a was no longer visible. 
On Friday,the tenth day, in addition to the usual increase in 
size of the cap, a very fine white line appeared on ca little be- 
low the ring, whose further history could not be traced. 
On Monday, October third, at five P. M., not quite fourteen 
days after the eggs were laid, the blastopore of a was closed. 
Within a few days the blastopores of the other developing eggs 
closed also. Within this week, owing perhaps to very hot 
weather, the eggs died. 
But they had lived long enough to uiri the idea of their 
exceedingly slow growth: fourteen days from the beginning of 
development to the closure of the blastopore, and no head eleva- 
tion, or primitive streak showing in this time. In all eggs 
where the primitive streak shows, or where the young embryo 
does not extend the full length of the egg, there is a cap over 
the egg, that extends about one millimeter beyond the tail of the 
embryo. This second cap, however, is caused by changes in the 
yolk. With the appearance and growth of the mesoderm, the 
yolk near the embryo and immediately beneath the shell be- 
comes spongy and fullof vacuoles in which lymph and blood 
spaces are laid out. This gives this part of the yolk a whiter 
appearance than the rest and so forms a cap. 
It is worthy of note that the five eggs that began to develop 
were supposedly all fertilized at the same time; nevertheless, 
there was a difference of from one to three days in the zes 
