Inclusion Criteria:

• Be at least 18 years of age
• Be surgically sterile or consent to use a medically acceptable method of contraception throughout the study period
• Have a histologically confirmed diagnosis of mSTS as defined by the 4 specified study subgrouped
• Have been treated in the metastatic setting with 0 to 2 prior systemic regimens for mSTS (Systemic treatment regimens given in the neoadjuvant setting and maintenance therapies will not be considered as regimens in the metastatic setting for the purposes of this protocol. Prior anthracycline-based regimen is allowable but not required. Subjects with extra-skeletal small round blue cell sarcomas, including rhabdomyosarcomas, must have exhausted or be intolerant of standard first line anthracycline-based chemotherapy.)
• Have measurable disease, as defined by RECIST v 1.1 assess within 2 weeks of study entry and have radiologically documented disease progression greater than or equal to a 10% increase in the sum of the longest diameters of target lesions present within 6 months prior to randomization
• Have tumor tissue available for TEM-1 biomarker studies
• Be willing and able to provide written informed consent

Exclusion Criteria:

• Have received more than 2 prior systemic treatment regimens for mSTS
• Have received either gemcitabine or docetaxel in any previous treatment for mSTS (regardless of the line of treatment)
• Have a diagnosis of primary bone sarcoma of any histological type.
• Have a history of clinically significant heart disease, or clinically significant arrhythmia on ECG within the past 6 months
• Have a history of allergic reaction to prior monoclonal antibody or biologic agent Have received previous treatment with MORAb-004 (anti-TEM-1)
• Have a medical condition with a high risk of bleeding (e.g., a known bleeding disorder, a coagulopathy, or a tumor that involves the major vessels) or have a recent (within past 6 months) history of a significant bleeding event
• Have undergone major surgical procedures or open biopsy, have significant traumatic injury within 30 days prior to the first date of study treatment, or have major surgical procedures anticipated during the study
• Have a serious non-healing wound, an ulcer (including gastrointestinal), or a bone fracture