EMBRYOLOGY OF BDELLODRILUS 207 
TABLE 4 
ENTODERM MESODERM NOT CERTAIN 
Crepidula | Amphitrite Dreissensia 
Nereis Arenicola Patella 
Podarke | Umbrella Spio 
Thalassema | Planorbis Serpulorbis 
Fiona Unio? Cyclas 
Ischnochiton | Limax | Aricia 
Physa fontanalis 
Physa hyponurum 
Aplysia 
the entoblast derived from M is greater in amount than the meso- 
derm, as found in Crepidula, and at the other extreme, where 
but two rudimentary cells of M are entoblastic, as in Aricia. 
According to Wilson, a series of this nature may indicate a 
gradual elimination of the entodermal element from the macro- 
mere D of the fourth quartette, and finally its complete trans- 
formation into the mesoblast. Kovalevksy (’71) suggested that 
this transformation shows quite forcibly that the mesoblast 
pole cells are to be regarded, phylogenetically, as derivatives of 
the archenteron, because of their close association with the 
posterior entoblast cell, D. 
The primary entoblasts, A, B, C and D, undergo but little 
change until late development in thcse forms which possess a 
larval stage, and may remain in this condition until after the 
trochophore is developed, or until after the blastopore is closed. 
In those individuals with a fetal type of development, they often 
remain distinct until after the germ bands are completely formed, 
as in Clepsine. 
GENERAL ADAPTATION AND INTERPRETATION OF CLEAVAGE 
The cleavage of eggs of widely separated forms exhibit unique 
resemblances. At certain stages of development these resem- 
blances exceed their differences. Is the persistence of these 
features due to the influence of ancestral inheritance, or are they 
due more to the adaptive conditions of their environment, to 
JOURNAL OF MORPHOLOGY, VOL. 26, No. 2 
