EXPLANATION OP PLATE ^— Continued, 



ElG. 93.— First cleavage-phase : the Ampliieytula has divided into two 

 cells, an upper, smaller, and clearer animal cell (mother-cell of the exoderm) ; 

 and a lower larger, and darker vegetative cell (mother-cell of the entoderm). 



Fig. 94. — Second cleavage-phase : the upper animal cell has divided into 

 two cells ; the lower vegetative cell is undivided. 



Fig. 95. — Third cleavage-phase : the upper animal cell is divided into 

 four cells by two meridianal grooves. The lower vegetative cell is still 

 uncleft. 



Fig. 96. — Later cleavage-phase : the upper animal cell has broken up 

 into several small clear cells ; the lower vegetative cell has divided into three 

 large dark cells (a lower larger and two upper smaller). 



Fig. 97. — Amphimorula. After the conclusion of the cleavage-process, 

 there is found above at the animal pole a hemispherical mass of numerous 

 small clear cells (exoderm) ; below, at the vegetative pole, a dark mass of a 

 few (six ?) large dark cells (entoderm). 



Fig. 98. — Amphiblastula in meridianal section. In the interior a cleavage- 

 cavity {s) has formed by accumulation of fluid ; in the upper (animal) half, 

 roofed in by a hemispherical layer of small clear exoderm-cells ; in the lower 

 (vegetative) half, closed by a few large dark entoderm-cells. 



Fig. 99. — Amphiblastula in the course of invagination or over-growth 

 (invaginata-circumcreta). The large dark entoderm-cells become pushed in 

 to the cleavage-cavity and eo ipso grown over by the clear exoderm-cells. 

 Optical meridianal section. 



Fig. 100. — Amphigastrula, in optical meridianal section. The invagina- 

 tion (entobole) or over-growth (epibole) of the Amphiblastula is finished, 

 the cleavage-cavity has disappeared, and the cavity of the archenteron is 

 formed, o, Urmund (blastopore). 



Fig. 101. — The same Amphigastrula, seen from the surface. 



Fig. 102. — Young worm-larva with a ring of cilia. 



Figs. 103. — 110. — Unequal cleavage and Amphigastrula of a Snail (Tro- 

 chus ?) 



Fig. 103. — Second cleavage-phase. Egg with four cleavage-cells. 



Fig. 104-. — Third cleavage-phase. Egg with eight cleavage-cells (four 

 small, clear animal cells, and four large, dark vegetative cells). 



Fig. 105. — The same egg in profile. 



Fig. 106. — Fourth . eleavage-phase. Egg with twelve cleavage-cells 

 (eight small, clear animal cells, and four large, dark vegetative cells). 



Fig. 107. — The same egg in profile. 



Fig. 108. — Amphimorula in meridianal section. After complete cleavage, 

 the upi)er animal hemisphere of the egg consists of sixteen small clear 

 cells ; the lower vegetative hemisphere, on the other hand, consists of eight 

 large dark cells (four upper larger, and four lower and smaller). 



Fig. 109. — Amphiblastula in meridianal section. The roof of the cleavage- 

 cavity is formed by thirty-two small clear cells, its floor by eight large 

 dark cells (four upper small, and four lower large). 



Fig. 110. — Amphigastrula in meridianal section. «, Urdarm (archenteron); 

 0, Urmund (blastopore). The exoderm (e) has completely grown over the 

 entoderm (?). Most of the cells of the last form the wall of the enteron. 

 A few large cells of the entoderm remain over and above as ' food-yelk ' {d). 



