"primitive streak" meaning in English

{
  "forms": [
    {
      "form": "primitive streaks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "primitive streak (plural primitive streaks)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Embryology",
          "orig": "en:Embryology",
          "parents": [
            "Biology",
            "Developmental biology",
            "Medicine",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A formation that occurs during the initial phases of embryonic development in birds, reptiles and mammals."
      ],
      "id": "en-primitive_streak-en-noun-OtXfYQIZ",
      "links": [
        [
          "embryology",
          "embryology"
        ],
        [
          "embryonic",
          "embryonic"
        ],
        [
          "birds",
          "birds"
        ],
        [
          "reptiles",
          "reptiles"
        ],
        [
          "mammals",
          "mammals"
        ]
      ],
      "raw_glosses": [
        "(embryology) A formation that occurs during the initial phases of embryonic development in birds, reptiles and mammals."
      ],
      "topics": [
        "embryology",
        "medicine",
        "sciences"
      ],
      "wikipedia": [
        "primitive streak"
      ]
    }
  ],
  "word": "primitive streak"
}

{
  "forms": [
    {
      "form": "primitive streaks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "primitive streak (plural primitive streaks)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Embryology"
      ],
      "glosses": [
        "A formation that occurs during the initial phases of embryonic development in birds, reptiles and mammals."
      ],
      "links": [
        [
          "embryology",
          "embryology"
        ],
        [
          "embryonic",
          "embryonic"
        ],
        [
          "birds",
          "birds"
        ],
        [
          "reptiles",
          "reptiles"
        ],
        [
          "mammals",
          "mammals"
        ]
      ],
      "raw_glosses": [
        "(embryology) A formation that occurs during the initial phases of embryonic development in birds, reptiles and mammals."
      ],
      "topics": [
        "embryology",
        "medicine",
        "sciences"
      ],
      "wikipedia": [
        "primitive streak"
      ]
    }
  ],
  "word": "primitive streak"
}