STUDIES IN SPICULE FORMATION. 309 
figs. 3 and 4). This internal budding commences as soon as 
the flattening of the posterior wall has become apparent and 
TExT-FIG. 1. TEXtT-FIe. 2. 
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continues up to the formation of the enteroccele or true meso- 
derm. It has been stated by Boveri (1) from observations 
made on the pigmented and unpigmented blastomeres of 
TEXT-FIG. 3. TEXT-FIG. 4. 
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Strongylocentrotus lividus that these free mesenchyme 
cells are all derived from that portion of the blastula wall 
which extends from the posterior pole as its centre upwards 
