MORPHOGENESIS IN
MICRASTERIAS
49
resting stage ; at germination it undergoes meiosis. However, in many
genera zygospores seem to occur rarely or have never been observed
(Heimans, 1942).
In young cells, as a rule, the chloroplast consists of a single continuous body provided with ridges and lobes characteristic of the species.
In cells ripe for division the original chloroplast divides in the region of
the nucleus. On cell division the chloroplast of the parent semicell
expands into the new growing semicell. Thus the chloroplast is an
organelle of structural continuity which does not arise de novo from
cytoplasmic elements; the contrary opinion, expressed by Teiling (1950)
as regards the constricted placodern desmids, does not agree with our
observations concerning Micrasterias. The various types of chloroplast
oscurring in desmids have been described in detail by Teiling (1952).
As in the Conjugales in general, the chloroplast contains pyrenoids and
forms starch.
Electron-microscopic studies of desmids have been made by Chardard
and Rouiller (1957), Drawert and Metzener-Küster (1961), Drawert and
Mix (1960, 1961, 1962a-c), and Waddington (1962). These workers have
elucidated the structure of cell wall, cytoplasm, nucleus, nucleolus,
chloroplast, pyrenoids and mitochondria, and have revealed the existence of a Golgi apparatus. In Micrasterias rotata Kiermayer and
Jarosch (1962) have observed Golgi bodies with the light microscope
and have found a connexion between them and the formation of
vacuoles.
III. Normal Features of Micrasterias
Owing to their large size and special features, many species of
Micrasterias are particularly suitable for morphogenetic studies. The
largest species may attain 200-300 μ or even more in length. With the
exception of the two asymmetrical species referred to earlier, the genus
is characterized by a disk-shaped cell divided by a median constriction
into two similar semicells (Fig. 1). The median constriction is due to the
sinus incision, and the narrow part, called the isthmus, is the normal
site of the nucleus at interphase. Each semicell is divided by incisions
into three major lobes, namely one polar lobe and two main lateral lobes
(lateral lobes in a wide sense, corresponding to the two wings). In the
large species, each main lateral lobe is dichotomously divided into smaller
lobes by incisions of different depths; the first incision separates the
upper and lower lateral lobes, the former being adjacent to the polar
lobe, the latter to the sinus. In typical cases, each of the outermost
lobules has a median depression and two angular dents at the end.
Owing to the dichotomous division of the lateral lobe, an even number
of lobules and dents can be regarded as representing the highest degree
MICRASTERIAS
49
resting stage ; at germination it undergoes meiosis. However, in many
genera zygospores seem to occur rarely or have never been observed
(Heimans, 1942).
In young cells, as a rule, the chloroplast consists of a single continuous body provided with ridges and lobes characteristic of the species.
In cells ripe for division the original chloroplast divides in the region of
the nucleus. On cell division the chloroplast of the parent semicell
expands into the new growing semicell. Thus the chloroplast is an
organelle of structural continuity which does not arise de novo from
cytoplasmic elements; the contrary opinion, expressed by Teiling (1950)
as regards the constricted placodern desmids, does not agree with our
observations concerning Micrasterias. The various types of chloroplast
oscurring in desmids have been described in detail by Teiling (1952).
As in the Conjugales in general, the chloroplast contains pyrenoids and
forms starch.
Electron-microscopic studies of desmids have been made by Chardard
and Rouiller (1957), Drawert and Metzener-Küster (1961), Drawert and
Mix (1960, 1961, 1962a-c), and Waddington (1962). These workers have
elucidated the structure of cell wall, cytoplasm, nucleus, nucleolus,
chloroplast, pyrenoids and mitochondria, and have revealed the existence of a Golgi apparatus. In Micrasterias rotata Kiermayer and
Jarosch (1962) have observed Golgi bodies with the light microscope
and have found a connexion between them and the formation of
vacuoles.
III. Normal Features of Micrasterias
Owing to their large size and special features, many species of
Micrasterias are particularly suitable for morphogenetic studies. The
largest species may attain 200-300 μ or even more in length. With the
exception of the two asymmetrical species referred to earlier, the genus
is characterized by a disk-shaped cell divided by a median constriction
into two similar semicells (Fig. 1). The median constriction is due to the
sinus incision, and the narrow part, called the isthmus, is the normal
site of the nucleus at interphase. Each semicell is divided by incisions
into three major lobes, namely one polar lobe and two main lateral lobes
(lateral lobes in a wide sense, corresponding to the two wings). In the
large species, each main lateral lobe is dichotomously divided into smaller
lobes by incisions of different depths; the first incision separates the
upper and lower lateral lobes, the former being adjacent to the polar
lobe, the latter to the sinus. In typical cases, each of the outermost
lobules has a median depression and two angular dents at the end.
Owing to the dichotomous division of the lateral lobe, an even number
of lobules and dents can be regarded as representing the highest degree