150
R. LALLIER
It has a central cavity, the blastocele. At the animal pole is a tuft of long,
stiff cilia. The change from blastula to gastrula begins with the invagination of material originating from the vegetal pole. The micromeres first
migrate into the blastocele where they later give rise to the skeleton.
Some of the blastomeres originating from the macromeres invaginate to
form a primitive digestive pocket, the archenteron. At this stage the
gastrula has a radial symmetry. The first signs of bilateral symmetry
appear with the flattening of one of the sides of the gastrula and its
transformation into an oral field surrounded by a ciliated zone. The
mouth of the pluteus is formed from an invagination of the ectoderm into
the oral zone. The stomodaeum next connects itself to the archenteron.
A pair of triradiate spicules is differentiated from the primary mesenchyme on the ventral side of the gastrula on both sides of the archenteron.
The transformation from gastrula to pluteus progresses by degrees
with the development of anal and ventral arms and the lengthening of
the larva, which will finally give the pluteus its characteristic form.
III. Experimental Study of Morphogenesis
A. Definitions
The sea urchin egg is extremely well suited to experimental interference such as may change the normal course of morphogenesis. The
relative proportions of the larval structures, either of entomesodermal
or ectodermal origins, can be considerably modified. Thus we can obtain
two distinct types of developmental deviation. They have been called
vegetalization and animalization respectively. Vegetalization corresponds to the hyperdevelopment of the entomesodermal larval structures.
Fig. 2d shows a vegetalized larva; it is characterized by a very large
exovaginated archenteron. The ectoderm, hardly developed, forms a
small thin-walled vesicle covered by short cilia. Animalization corresponds to the hyperdevelopment of the ectodermal structures. Fig. 2c
shows an animalized larva; the well-developed ectoderm forms a
voluminous ciliated vesicle ; a large tuft of cilia covers the animal pole ;
the archenteron is not differentiated. The larvae described correspond to
ones very strongly vegetalized and animalized. All stages intermediate
between extreme animalization and vegetalization can be obtained. To
these can be added a third type of modification of development:
radialization. This corresponds to the formation of a larval type having
a radial symmetry. Fig. 2e shows a radialized larva. It is characterized
by the axial orientation of the archenteron, the arrangement of the
spicules which form a crown around the archenteron base, and the
lengthening of the pre-oral lobe. These larvae can easily be obtained by
treating the eggs with some animalizing agents in weak concentration.
R. LALLIER
It has a central cavity, the blastocele. At the animal pole is a tuft of long,
stiff cilia. The change from blastula to gastrula begins with the invagination of material originating from the vegetal pole. The micromeres first
migrate into the blastocele where they later give rise to the skeleton.
Some of the blastomeres originating from the macromeres invaginate to
form a primitive digestive pocket, the archenteron. At this stage the
gastrula has a radial symmetry. The first signs of bilateral symmetry
appear with the flattening of one of the sides of the gastrula and its
transformation into an oral field surrounded by a ciliated zone. The
mouth of the pluteus is formed from an invagination of the ectoderm into
the oral zone. The stomodaeum next connects itself to the archenteron.
A pair of triradiate spicules is differentiated from the primary mesenchyme on the ventral side of the gastrula on both sides of the archenteron.
The transformation from gastrula to pluteus progresses by degrees
with the development of anal and ventral arms and the lengthening of
the larva, which will finally give the pluteus its characteristic form.
III. Experimental Study of Morphogenesis
A. Definitions
The sea urchin egg is extremely well suited to experimental interference such as may change the normal course of morphogenesis. The
relative proportions of the larval structures, either of entomesodermal
or ectodermal origins, can be considerably modified. Thus we can obtain
two distinct types of developmental deviation. They have been called
vegetalization and animalization respectively. Vegetalization corresponds to the hyperdevelopment of the entomesodermal larval structures.
Fig. 2d shows a vegetalized larva; it is characterized by a very large
exovaginated archenteron. The ectoderm, hardly developed, forms a
small thin-walled vesicle covered by short cilia. Animalization corresponds to the hyperdevelopment of the ectodermal structures. Fig. 2c
shows an animalized larva; the well-developed ectoderm forms a
voluminous ciliated vesicle ; a large tuft of cilia covers the animal pole ;
the archenteron is not differentiated. The larvae described correspond to
ones very strongly vegetalized and animalized. All stages intermediate
between extreme animalization and vegetalization can be obtained. To
these can be added a third type of modification of development:
radialization. This corresponds to the formation of a larval type having
a radial symmetry. Fig. 2e shows a radialized larva. It is characterized
by the axial orientation of the archenteron, the arrangement of the
spicules which form a crown around the archenteron base, and the
lengthening of the pre-oral lobe. These larvae can easily be obtained by
treating the eggs with some animalizing agents in weak concentration.