DEVELOPMENT OF SIMOCEPHALUS 631 
a vitelline membrane if this term is restricted, as McMurrich 
(10) maintains, to a membrane which is connected with the 
process of fertilization, but must be termed a primary egg- 
membrane or ‘ Dotterhaut’ as defined by Korschelt and 
Heider (7). 
4. CLEAVAGE. 
Cleavage is completely superficial. At first the separate 
blastomeres remain deep in the egg as apparently amoeboid 
masses of protoplasm. After five hours they begin to appear 
on the surface, and soon after, each blastomere becomes 
separated from its neighbours by furrows extending a short 
distance into the yolk. Hight hours after the egg has been 
laid cleavage is complete and results in a uniform blastoderm 
enclosing the yolk-mass. No yolk-cells were found in the 
interior of the blastula. In Daphnia similis Lebedinski (9) 
found that certain blastomeres remained behind in the centre 
of the egg while the remainder migrated towards its surface 
to form the blastoderm, and that the former blastomeres 
functioned in absorbing the fat or yolk-drops. Vollmer (14) 
in the winter eggs of Cladocera describes the formation of 
a blastula with greatly reduced blastocoele by total cleavage, 
and states that cells are budded off from the blastomeres into 
the interior of the egg which function as yolk-cells. In 
Leptodora hyalina Samter (12) found that yolk-cells 
were budded off from the blastoderm at the same time that 
the endoderm plate commenced to immigrate into the egg. 
Agar (1) in Holopedium gibberum states that ‘ fairly 
late stages show occasional very flat nuclei on the separate 
yolk-masses, as figured by Samassa (11). Deubtless each 
yolk-mass is contained in a single cell. The origin of these 
yolk-cells has not been observed, but it may be safely assumed 
that they arise in the same way as that described by Samassa, 
i.e. by budding off from the mesendoderm’. Similarly, in 
S. vetulus embryos in which the endoderm jhas already 
separated from the mesoderm often contain yolk-masses 
against which flattened cells are seen to be lying. Their origin 
