primitive streak

…The method of gastrulation in the cephalochordates (lampfish), fish, amphibians, and reptiles can be considered as invagination and overgrowth, or a modified form of it (Fig. 3-c), where the proportion of presumptive endoderm in the gastrula wall decreases, and its dorsal part (the gastrula lid) is mainly occupied by presumptive mesoderm cells. On the other hand, in birds and mammals, the cells of the blastoderm (birds) or inner cell mass (mammals), which corresponds to the blastula, first divide into two layers, an inner and an outer layer, and then a long and thin thickened part called the primitive streak appears in the outer layer (presumptive ectoderm and presumptive mesoderm), from which a large number of cells spill into the gap between the two layers (corresponding to the blastocoel), and the three germ layers differentiate (Fig. 3-d). The anterior region of this primitive streak and the gastrula lid are called the organizer, and active morphogenetic movements soon take place in this area. …

*Some of the terminology explanations that mention "primitive streak" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…原索動物の頭索類(ナメクジウオ)，魚類，両生類，爬虫類における原腸形成の方法は，陥入およびおおいかぶせ，またはその変形とみなすことができる(図3‐c)が，その際，原腸壁に占める予定内胚葉の割合は小さくなり，その背側部(原腸蓋)は主として予定中胚葉細胞の占めるところとなる。一方，鳥類および哺乳類においては，胞胚に相当する胚盤葉(鳥類)または内部細胞塊(哺乳類)の細胞がまず内外2層に分かれ，続いてその外層(予定外胚葉と予定中胚葉)に原条primitive streakとよばれる細長い肥厚部が出現し，そこから多数の細胞が両層の間隙（かんげき）(胞胚腔に相当する)にこぼれ落ちることによって，三つの胚葉が分化する(図3‐d)。　この原条の前方域および原腸蓋は形成体organizerとよばれ，まもなくその部位では活発な形態形成運動がくり広げられる。…

※「primitive streak」について言及している用語解説の一部を掲載しています。

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